OF PROBABILITY, AND OF THE IDEA OF CAUSE AND EFFECT.
This is all I think necessary to observe concerning those four
relations, which are the foundation of science; but as to the other
three, which depend not upon the idea, and may be absent or present
even while _that_ remains the same, 'twill be proper to explain them
more particularly. These three relations are _identity, the situations
in time and place, and causation_.
All kinds of reasoning consist in nothing but a _comparison_, and a
discovery of those relations, either constant or inconstant, which two
or more objects bear to each other. This comparison we may make, either
when both the objects are present to the senses, or when neither of
them is present, or when only one. When both the objects are present to
the senses along with the relation, we call _this_ perception rather
than reasoning; nor is there in this case any exercise of the thought,
or any action, properly speaking, but a mere passive admission of
the impressions through the organs of sensation. According to this
way of thinking, we ought not to receive as reasoning any of the
observations we may make concerning _identity_, and the _relations_
of _time_ and _place_; since in none of them the mind can go beyond
what is immediately present to the senses, either to discover the real
existence or the relations of objects. 'Tis only _causation_, which
produces such a connexion, as to give us assurance from the existence
or action of one object, that 'twas followed or preceded by any other
existence or action; nor can the other two relations ever be made use
of in reasoning, except so far as they either affect or are affected
by it. There is nothing in any objects to persuade us, that they are
either always _remote_ or always _contiguous_; and when from experience
and observation we discover, that their relation in this particular
is invariable, we always conclude there is some secret _cause_ which
separates or unites them. The same reasoning extends to _identity_. We
readily suppose an object may continue individually the same, though
several times absent from and present to the senses; and ascribe to
it an identity, notwithstanding the interruption of the perception,
whenever we conclude, that if we had kept our eye or hand constantly
upon it, it would have conveyed an invariable and uninterrupted
perception. But this conclusion beyond the impressions of our senses
can be founded only on the connexion of _cause and effect_; nor can
we otherwise have any security that the object is not changed upon
us, however much the new object may resemble that which was formerly
present to the senses. Whenever we discover such a perfect resemblance,
we consider whether it be common in that species of objects; whether
possibly or probably any cause could operate in producing the change
and resemblance; and according as we determine concerning these causes
and effects, we form our judgment concerning the identity of the object.
Here then it appears, that of those three relations, which depend not
upon the mere ideas, the only one that can be traced beyond our senses,
and informs us of existences and objects, which we do not see or feel,
is _causation_. This relation therefore we shall endeavour to explain
fully before we leave the subject of the Of the understanding.
To begin regularly, we must consider the idea of _causation_, and see
from what origin it is derived. 'Tis impossible to reason justly,
without understanding perfectly the idea concerning which we reason;
and 'tis impossible perfectly to understand any idea, without tracing
it up to its origin, and examining that primary impression, from which
it arises. The examination of the impression bestows a clearness on the
idea; and the examination of the idea bestows a like clearness on all
our reasoning.
Let us therefore cast our eye on any two objects, which we call
cause and effect, and turn them on all sides, in order to find that
impression, which produces an idea of such prodigious consequence.
At first sight I perceive, that I must not search for it in any of
the particular _qualities_ of the objects; since, whichever of these
qualities I pitch on, I find some object that is not possessed of it,
and yet falls under the denomination of cause or effect. And indeed
there is nothing existent, either externally or internally, which is
not to be considered either as a cause or an effect; though 'tis plain
there is no one quality which universally belongs to all beings, and
gives them a title to that denomination.
The idea then of causation must be derived from some _relation_ among
objects; and that relation we must now endeavour to discover. I find
in the first place, that whatever objects are considered as causes or
effects, are _contiguous_; and that nothing can operate in a time or
place, which is ever so little removed from those of its existence.
Though distant objects may sometimes seem productive of each other,
they are commonly found upon examination to be linked by a chain of
causes, which are contiguous among themselves, and to the distant
objects; and when in any particular instance we cannot discover this
connexion, we still presume it to exist. We may therefore consider the
relation of _contiguity_ as essential to that of causation; at least
may suppose it such, according to the general opinion, till we can find
a more proper occasion[2] to clear up this matter, by examining what
objects are or are not susceptible of juxtaposition and conjunction.
The second relation I shall observe as essential to causes and effects,
is not so universally acknowledged, but is liable to some controversy.
'Tis that of _priority_ of time in the cause before the effect. Some
pretend that 'tis not absolutely necessary a cause should precede its
effect; but that any object or action, in the very first moment of its
existence, may exert its productive quality, and give rise to another
object or action, perfectly cotemporary with itself. But beside that
experience in most instances seems to contradict this opinion, we may
establish the relation of priority by a kind of inference or reasoning.
'Tis an established maxim both in natural and moral philosophy, that
an object, which exists for any time in its full perfection without
producing another, is not its sole cause; but is assisted by some other
principle which pushes it from its state of inactivity, and makes it
exert that energy, of which it was secretly possessed. Now if any cause
may be perfectly cotemporary with its effect, 'tis certain, according
to this maxim, that they must all of them be so; since any one of
them, which retards its operation for a single moment, exerts not
itself at that very individual time, in which it might have operated;
and therefore is no proper cause. The consequence of this would be
no less than the destruction of that succession of causes, which we
observe in the world; and indeed the utter annihilation of time. For
if one cause were cotemporary with its effect, and this effect with
_its_ effect, and so on, 'tis plain there would be no such thing as
succession, and all objects must be co-existent.
If this argument appear satisfactory, 'tis well. If not, I beg the
reader to allow me the same liberty, which I have used in the preceding
case, of supposing it such. For he shall find, that the affair is of no
great importance.
Having thus discovered or supposed the two relations of _contiguity_
and _succession_ to be essential to causes and effects, I find I am
stopped short, and can proceed no farther in considering any single
instance of cause and effect. Motion in one body is regarded upon
impulse as the cause of motion in another. When we consider these
objects with the utmost attention, we find only that the one body
approaches the other; and that the motion of it precedes that of
the other, but without any sensible interval. 'Tis in vain to rack
ourselves with _farther_ thought and reflection upon this subject. We
can go no _farther_ in considering this particular instance.
Should any one leave this instance, and pretend to define a cause, by
saying it is something productive of another, 'tis evident he would
say nothing. For what does he mean by _production_? Can he give any
definition of it, that will not be the same with that of causation?
If he can, I desire it maybe produced. If he cannot, he here runs in a
circle, and gives a synonymous term instead of a definition.
Shall we then rest contented with these two relations of contiguity and
succession, as affording a complete idea of causation? By no means. An
object may be contiguous and prior to another, without being considered
as its cause. There is a _necessary connexion_ to be taken into
consideration; and that relation is of much greater importance, than
any of the other two above mentioned.
Here again I turn the object on all sides, in order to discover the
nature of this necessary connexion, and find the impression, or
impressions, from which its idea may be derived. When I cast my eye
on the _known qualities_ of objects, I immediately discover that the
relation of cause and effect depends not in the least on _them_. When
I consider their _relations_, I can find none but those of contiguity
and succession; which I have already regarded as imperfect and
unsatisfactory. Shall the despair of success make me assert, that I
am here possessed of an idea, which is not preceded by any similar
impression? This would be too strong a proof of levity and inconstancy;
since the contrary principle has been already so firmly established,
as to admit of no farther doubt; at least, till we have more fully
examined the present difficulty.
We must therefore proceed like those who, being in search of any thing
that lies concealed from them, and not finding it in the place they
expected, beat about all the neighbouring fields, without any certain
view or design, in hopes their good fortune will at last guide them to
what they search for. 'Tis necessary for us to leave the direct survey
of this question concerning the nature of that _necessary connexion_,
which enters into our idea of cause and effect; and endeavour to find
some other questions, the examination of which will perhaps afford
a hint, that may serve to clear up the present difficulty. Of these
questions there occur two, which I shall proceed to examine, viz.
First, for what reason we pronounce it _necessary_, that every thing
whose existence has a beginning, should also have a cause?
Secondly, why we conclude, that such particular causes must
_necessarily_ have such particular effects; and what is the nature of
that _inference_ we draw from the one to the other, and of the _belief_
we repose in it?
I shall only observe before I proceed any farther, that though the
ideas of cause and effect be derived from the impressions of reflection
as well as from those of sensation, yet for brevity's sake, I commonly
mention only the latter as the origin of these ideas; though I desire
that, whatever I say of them, may also extend to the former. Passions
are connected with their objects and with one another; no less than
external bodies are connected together. The same relation then of cause
and effect, which belongs to one, must be common to all of them.
[2] Part IV. Sect 5.