In the case of the smell of burning, when we find on looking about, that
something actually is on fire, we say indifferently either that we have
found out the cause of the smell, or that we know the =reason why= we
perceive that smell; or that we have =explained= it. So that to know the
reason why of anything, or to explain it, is to know the cause of it.
But that which is the cause of one thing is the effect of another. Thus,
suppose we find some smouldering straw to be the cause of the smell of
burning, we immediately ask what set it on fire, or what is the cause of
its burning? Perhaps we find that a lighted lucifer match has been
thrown into the straw, and then we say that the lighted match was the
cause of the fire. But a lucifer match would not be in that place unless
some person had put it there. That is to say, the presence of the
lucifer match is an effect produced by somebody as cause. So we ask why
did any one put the match there? Was it done carelessly, or did the
person who put it there intend to do so? And if so, what was his motive,
or the cause which led him to do such a thing? And what was the reason
for his having such a motive? It is plain that there is no end to the
questions, one arising out of the other, that might be asked in this
fashion.
Thus we believe that everything is the effect of something which
preceded it as its cause, and that this cause is the effect of something
else, and so on, through a chain of causes and effects which goes back
as far as we choose to follow it. Anything is said to be explained as
soon as we have discovered its cause, or the reason why it exists; the
explanation is fuller, if we can find out the cause of that cause; and
the further we can trace the chain of causes and effects, the more
satisfactory is the explanation. But no explanation of anything can be
complete, because human knowledge, at its best, goes but a very little
way back towards the beginning of things.