With all _transcendental_ ideality the objective world retains _empirical_
reality; the object is indeed not the thing in itself, but as an empirical
object it is real. It is true that space is only in my head; but
empirically my head is in space. The law of causality can certainly never
enable us to get quit of idealism by building a bridge between things in
themselves and our knowledge of them, and thus certifying the absolute
reality of the world, which exhibits itself in consequence of its
application; but this by no means does away with the causal relation of
objects to each other, thus it does not abolish the causal relation which
unquestionably exists between the body of each knowing person and all
other material objects. But the law of causality binds together only
phenomena, and does not lead beyond them. With that law we are and remain
in the world of objects, _i.e._, the world of phenomena, or more properly
the world of ideas. Yet the whole of such a world of experience is
primarily conditioned by the knowledge of a subject in general as its
necessary presupposition, and then by the special forms of our perception
and apprehension, thus necessarily belongs to the merely _phenomenal_, and
has no claim to pass for the world of things in themselves. Indeed the
subject itself (so far as it is merely the _knowing_ subject) belongs to
the merely phenomenal, of which it constitutes the complementary half.
Without application of the law of causality, however, perception of an
_objective_ world could never be arrived at; for this perception is, as I
have often explained, essentially matter of the _intellect_, and not
merely of the _senses_. The senses afford us mere _sensation_, which is
far from being _perception_. The part played by sensations of the senses
in perception was distinguished by Locke under the name _secondary
qualities_, which he rightly refused to ascribe to things in themselves.
But Kant, carrying Locke’s method further, distinguished also, and refused
to ascribe to things in themselves what belongs to the working up of this
material (the sensations) by the brain. The result was, that in this was
included all that Locke had left to things in themselves as _primary_
qualities—extension, form, solidity, &c.—so that with Kant the thing in
itself was reduced to a completely unknown quantity = _x_. With Locke
accordingly the thing in itself is certainly without colour, sound, smell,
taste, neither warm nor cold, neither soft nor hard, neither smooth nor
rough; yet it has still extension and form, it is impenetrable, at rest or
in motion, and has mass and number. With Kant, on the other hand, it has
laid aside all these latter qualities also, because they are only possible
by means of time, space, and causality, and these spring from an intellect
(brain), just as colours, tones, smells, &c., originate in the nerves of
the organs of sense. The thing in itself has with Kant become spaceless,
unextended, and incorporeal. Thus what the mere senses bring to the
perception, in which the objective world exists, stands to what is
supplied by the _functions of the brain_ (space, time, causality) as the
mass of the nerves of sense stand to the mass of the brain, after
subtracting that part of the latter which is further applied to thinking
proper, _i.e._, to abstract ideas, and is therefore not possessed by the
brutes. For as the nerves of the organs of sense impart to the phenomenal
objects colour, sound, taste, smell, temperature, &c., so the brain
imparts to them extension, form, impenetrability, the power of movement,
&c., in short all that can only be presented in perception by means of
time, space, and causality. How small is the share of the senses in
perception, compared with that of the intellect, is also shown by a
comparison of the nerve apparatus for receiving impressions with that for
working them up. The mass of the nerves of sensation of the whole of the
organs of sense is very small compared with that of the brain, even in the
case of the brutes, whose brain, since they do not, properly speaking,
_i.e._, in the abstract, think, is merely used for effecting perception,
and yet when this is complete, thus in the case of mammals, has a very
considerable mass, even after the cerebellum, whose function is the
systematic guidance of movements, has been taken away.
That excellent book by Thomas Reid, the “Inquiry into the Human Mind”
(first edition, 1764; 6th edition, 1810), as a _negative_ proof of the
Kantian truths, affords us a very thorough conviction of the inadequacy of
the senses to produce the objective perception of things, and also of the
non-empirical origin of the perception of space and time. Reid refutes
Locke’s doctrine that perception is a product of the senses, by a thorough
and acute demonstration that the collective sensations of the senses do
not bear the least resemblance to the world as known in perception, and
especially that the five primary qualities of Locke (extension, form,
solidity, movement, and number) absolutely could not be afforded us by any
sensation of the senses. Accordingly he gives up the question as to the
mode of origination and the source of perception as completely insoluble;
and although altogether unacquainted with Kant, he gives us, as it were,
according to the _regula falsi_, a thorough proof of the intellectual
nature of perception (really first explained by me as a consequence of the
Kantian doctrine), and also of the _a priori_ source, discovered by Kant,
of its constituent elements, space, time, and causality, from which those
primary qualities of Locke first proceed, but by means of which they are
easily constructed. Thomas Reid’s book is very instructive and well worth
reading—ten times more so than all the philosophy together that has been
written since Kant. Another indirect proof of the same doctrine, though in
the way of error, is afforded by the French sensational philosophers, who,
since Condillac trod in the footsteps of Locke, have laboured to show once
for all that the whole of our perception and thinking can be referred to
mere sensations (_penser c’est sentir_), which, after Locke’s example,
they call _idées simples_, and through the mere coming together and
comparison of which the whole objective world is supposed to build itself
up in our heads. These gentlemen certainly have _des idées bien simples_.
It is amusing to see how, lacking alike the profundity of the German and
the honesty of the English philosopher, they turn the poor material of
sensation this way and that way, and try to increase its importance, in
order to construct out of it the deeply significant phenomena of the world
of perception and thought. But the man constructed by them would
necessarily be an _Anencephalus_, a _Tête de crapaud_, with only organs of
sense and without a brain. To take only a couple of the better attempts of
this sort out of a multitude of others, I may mention as examples
Condorcet at the beginning of his book, “_Des Progrès de l’Esprit
Humain_,” and Tourtual on Sight, in the second volume of the
_“__Scriptures Ophthalmologici Minores,__”__ edidit Justus Radius_ (1828).
The feeling of the insufficiency of a purely sensationalistic explanation
of perception is in like manner shown in the assertion which was made
shortly before the appearance of the Kantian philosophy, that we not only
have _ideas_ of things called forth by sensation, but apprehend the
_things themselves_ directly, although they lie outside us—which is
certainly inconceivable. And this was not meant in some idealistic sense,
but was said from the point of view of common realism. This assertion is
well and pointedly put by the celebrated Euler in his “Letters to a German
Princess,” vol. ii. p. 68. He says: “I therefore believe that the
sensations (of the senses) contain something more than philosophers
imagine. They are not merely empty perceptions of certain impressions made
in the brain. They do not give the soul mere _ideas_ of things, _but
actually place before it objects_ which exist outside it, although we
cannot conceive how this really happens.” This opinion is explained by the
following facts. Although, as I have fully proved, perception is brought
about by application of the law of causality, of which we are conscious _a
priori_, yet in sight the act of the understanding, by means of which we
pass from the effect to the cause, by no means appears distinctly in
consciousness; and therefore the sensation does not separate itself
clearly from the idea which is constructed out of it, as the raw material,
by the understanding. Still less can a distinction between object and
idea, which in general does not exist, appear in consciousness; but we
feel the _things themselves_ quite directly, and indeed as lying _outside
us_, although it is certain that what is immediate can only be the
sensation, and this is confined to the sphere of the body enclosed by our
skin. This can be explained from the fact that _outside us_ is exclusively
a _spatial_ determination. But space itself is a form of our faculty of
perception, _i.e._, a function of our brain. Therefore that externality to
us to which we refer objects, on the occasion of sensations of sight, is
itself really within our heads; for that is its whole sphere of activity.
Much as in the theatre we see the mountains, the woods, and the sea, but
yet everything is inside the house. From this it becomes intelligible that
we perceive things in the relation of externality, and yet in every
respect immediately, but have not within us an idea of the things which
lie outside us, different from these things. For things are in space, and
consequently also external to us only in so far as we _perceive_ them.
Therefore those things which to this extent we perceive directly, and not
mere images of them, are themselves only _our ideas_, and as such exist
only in our heads. Therefore we do not, as Euler says, directly perceive
the things themselves which are external to us, but rather the things
which are perceived by us as external to us are only our ideas, and
consequently are apprehended by us immediately. The whole observation
given above in Euler’s words, and which is quite correct, affords a fresh
proof of Kant’s Transcendental Æsthetic, and of my theory of perception
which is founded upon it, as also of idealism in general. The directness
and unconsciousness referred to above, with which in perception we make
_the transition from the sensation to its cause_, may be illustrated by an
analogous procedure in the use of abstract ideas or thinking. When we read
or hear we receive mere words, but we pass from these so immediately to
the conceptions denoted by them, that it is as if we _received the
conceptions directly_; for we are absolutely unconscious of the transition
from the words to the conceptions. Therefore it sometimes happens that we
do not know in what language it was that we read something yesterday which
we now remember. Yet that such a transition always takes place becomes
apparent if it is once omitted, that is, if in a fit of abstraction we
read without thinking, and then become aware that we certainly have taken
in all the words but no conceptions. Only when we pass from abstract
conceptions to pictures of the imagination do we become conscious of the
transposition we have made.
Further, it is really only in perception in the narrowest sense, that is,
in _sight_, that in empirical apprehension the transition from the
sensation to its cause takes place quite unconsciously. In every other
kind of sense perception, on the contrary, the transition takes place with
more or less distinct consciousness; therefore, in the case of
apprehension through the four coarser senses, its reality is capable of
being established as an immediate fact. Thus in the dark we feel a thing
for a long time on all sides until from the different effects upon our
hands we are able to construct its definite form as their cause. Further,
if something feels smooth we sometimes reflect whether we may not have fat
or oil upon our hands; and again, if something feels cold we ask ourselves
whether it may not be that we have very warm hands. When we hear a sound
we sometimes doubt whether it was really an affection of our sense of
hearing from without or merely an inner affection of it; then whether it
sounded near and weak or far off and strong, then from what direction it
came, and finally whether it was the voice of a man or of a brute, or the
sound of an instrument; thus we investigate the cause of each effect we
experience. In the case of smell and taste uncertainty as to the objective
nature of the cause of the effect felt is of the commonest occurrence, so
distinctly are the two separated here. The fact that _in sight_ the
transition from the effect to the cause occurs quite unconsciously, and
hence the illusion arises that this kind of perception is perfectly
direct, and consists simply in the sensation alone without any operation
of the understanding—this has its explanation partly in the great
perfection of the organ of vision, and partly in the exclusively
rectilineal action of light. On account of the latter circumstance the
impression itself leads directly to the place of the cause, and since the
eye is capable of perceiving with the greatest exactness and at a glance
all the fine distinctions of light and shade, colour and outline, and also
the data in accordance with which the understanding estimates distance, it
thus happens that in the case of impressions of this sense the operation
of the understanding takes place with such rapidity and certainty that we
are just as little conscious of it as of spelling when we read. Hence
arises the delusion that the sensation itself presents us directly with
the objects. Yet it is just in sight that the operation of the
_understanding_, consisting in the knowledge of the cause from the effect,
is most significant. By means of it what is felt doubly, with two eyes, is
perceived as single; by means of it the impression which strikes the
retina upside down, in consequence of the crossing of the rays in the
pupils, is put right by following back the cause of this in the same
direction, or as we express ourselves, we see things upright although
their image in the eye is reversed; and finally by means of the operation
of the understanding magnitude and distance are estimated by us in direct
perception from five different data, which are very clearly and
beautifully described by Dr. Thomas Reid. I expounded all this, and also
the proofs which irrefutably establish the _intellectual nature of
perception_, as long ago as 1816, in my essay “On Sight and Colour”
(second edition, 1854; third edition, 1870), and with important additions
fifteen years later in the revised Latin version of it which is given
under the title, “_Theoria Colorum Physiologica Eademque Primaria_,” in
the third volume of the “_Scriptores Ophthalmologici Minores_,” published
by Justus Radius in 1830; yet most fully and thoroughly in the second (and
third) edition of my essay “On the Principle of Sufficient Reason,” § 21.
Therefore on this important subject I refer to these works, so as not to
extend unduly the present exposition.
On the other hand, an observation which trenches on the province of
æsthetics may find its place here. It follows from the proved intellectual
nature of perception that the sight of beautiful objects—for example, of a
beautiful view—is also a _phenomenon of the brain_. Its purity and
completeness, therefore, depends not merely on the object, but also upon
the quality of the brain, its form and size, the fineness of its texture,
and the stimulation of its activity by the strength of the pulse of the
arteries which supply it. Accordingly the same view appears in different
heads, even when the eyes are equally acute, as different as, for example,
the first and last impressions of a copper plate that has been much used.
This is the explanation of the difference of capacity for enjoying natural
beauty, and consequently also for reproducing it, _i.e._, for occasioning
a similar phenomenon of the brain by means of an entirely different kind
of cause, the arrangement of colours on a canvas.
The apparent immediacy of perception, depending on its entire
intellectuality, by virtue of which, as Euler says, we apprehend the thing
itself, and as external to us, finds an analogy in the way in which we
feel the parts of our own bodies, especially when they suffer pain, which
when we do feel them is generally the case. Just as we imagine that we
perceive things where they are, while the perception really takes place in
the brain, we believe that we feel the pain of a limb in the limb itself,
while in reality it also is felt in the brain, to which it is conducted by
the nerve of the affected part. Therefore, only the affections of those
parts whose nerves go to the brain are felt, and not those of the parts
whose nerves belong to the sympathetic system, unless it be that an
unusually strong affection of these parts penetrates by some roundabout
way to the brain, where yet for the most part it only makes itself known
as a dull sense of discomfort, and always without definite determination
of its locality. Hence, also, it is that we do not feel injuries to a limb
whose nerve-trunk has been severed or ligatured. And hence, finally, the
man who has lost a limb still sometimes feels pain in it, because the
nerves which go to the brain are still there. Thus, in the two phenomena
here compared, what goes on in the brain is apprehended as outside of it;
in the case of perception, by means of the understanding, which extends
its feelers into the outer world; in the case of the feeling of our limbs,
by means of the nerves.