ONE:387It is well known that Spinoza draws a sharp line of demarcation between the two attributes of Extension and Thought, which, with him, correspond to what are usually called body and mind. Neither attribute can act on the other. Mind receives no impressions from body, nor does body receive any impulses from mind. This proposition follows by rigorous logical necessity from the Platonic principle that mind is independent of body, combined with the Stoic principle that nothing but body can act on body, generalised into the wider principle that interaction implies homogeneity of nature. According to some critics, Spinoza��s teaching on this point constitutes a fatal flaw in his philosophy. How, it is asked, can we know that there is any such thing as body (or extension) if body cannot be perceived,��for perceived it certainly cannot be without acting on our minds? The idea of infinite substance suggests a way out of the408 difficulty. ��I find in myself,�� Spinoza might say, ��the idea of extension. In fact, my mind is nothing but the idea of extension, or the idea of that idea, and so on through as many self-reflections as you please. At the same time, mind, or thought, is not itself extended. Descartes and the Platonists before him have proved thus much. Consequently I can conceive extension as existing independently of myself, and, more generally, of all thought. But how can I be sure that it actually does so exist? In this wise. An examination of thought leads me to the notion of something in which it resides��a substance whose attribute it is. But having once conceived such a substance, I cannot limit it to a single attribute, nor to two, nor to any finite number. Limitation implies a boundary, and there can be no boundary assigned to existence, for existence by its very definition includes everything that is. Accordingly, whatever can be conceived, in other words whatever can be thought without involving a contradiction,��an important reservation which I beg you to observe,��must necessarily exist. Now extension involves no contradiction, therefore it exists,��exists, that is to say, as an attribute of the infinite substance. And, by parity of reasoning, there must be an idea of extension; for this also can exist without involving a contradiction, as the simplest introspection suffices to show. You ask me why then I do not believe in gorgons and chimaeras. I answer that since, in point of fact, they do not exist, I presume that their notion involves a contradiction, although my knowledge of natural law is not sufficiently extended to show me where the contradiction lies. But perhaps science will some day be able to point out in every instance of a non-existing thing, where the contradiction lies, no less surely than it can now be pointed out in the case of impossible geometrical figures.�� In short, while other people travel straight from their sensations to an external world, Spinoza travels round to it by the idea of an infinite substance.564