The dogma of the immortality of the soul assumes that the soul is a
simple substance, a spirit; but I will always ask, what is a spirit? It
is, you say, a substance deprived of expansion, incorruptible, and which
has nothing in common with matter. But if this is true, how came your
soul into existence? how did it grow? how did it strengthen? how weaken
itself, get out of order, and grow old with your body? In reply to all
these questions, you say that they are mysteries; but if they are
mysteries, you understand nothing about them. If you do not understand
anything about them, how can you positively affirm anything about them?
In order to believe or to affirm anything, it is necessary at least to
know what that consists of which we believe and which we affirm. To
believe in the existence of your immaterial soul, is to say that you are
persuaded of the existence of a thing of which it is impossible for you
to form any true idea; it is to believe in words without attaching any
sense to them; to affirm that the thing is as you claim, is the highest
folly or assumption.