136. OBJECTION.--ANSWER.--It will perhaps be said that we want a sense
(as some have imagined) proper to know substances withal, which,
if we had, we might know our own soul as we do a triangle. To this
I answer, that, in case we had a new sense bestowed upon us, we
could only receive thereby some new sensations or ideas of sense.
But I believe nobody will say that what he means by the terms soul
and substance is only some particular sort of idea or sensation.
We may therefore infer that, all things duly considered, it is
not more reasonable to think our faculties defective, in that they do not
furnish us with an idea of spirit or active thinking substance, than it
would be if we should blame them for not being able to comprehend a round
square.

137. From the opinion that spirits are to be known after the manner of an
idea or sensation have risen many absurd and heterodox tenets, and much
scepticism about the nature of the soul. It is even probable that this
opinion may have produced a doubt in some whether they had any soul at
all distinct from their body since upon inquiry they could not find they
had an idea of it. That an idea which is inactive, and the existence
whereof consists in being perceived, should be the image or likeness of
an agent subsisting by itself, seems to need no other refutation than
barely attending to what is meant by those words. But, perhaps you will
say that though an idea cannot resemble a spirit in its thinking, acting,
or subsisting by itself, yet it may in some other respects; and it is not
necessary that an idea or image be in all respects like the original.

138. I answer, if it does not in those mentioned, it is impossible it
should represent it in any other thing. Do but leave out the power of
willing, thinking, and perceiving ideas, and there remains nothing else
wherein the idea can be like a spirit. For, by the word spirit we mean