understanding that it does not perceive the idea of spirit, if it is manifestly impossible there should be any such idea.  And this if I mistake not has been demonstrated in section 27; to which I shall here add that a spirit has been shown to be the only substance or support wherein unthinking beings or ideas can exist; but that this substance which supports or perceives ideas should itself be an idea or like an idea is evidently absurd.

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 only that which thinks, wills, and perceives; this, and this alone, constitutes the signification of the term.  If therefore it is impossible that any degree of those powers should be represented in an idea, it is evident there can be no idea of a spirit.