The sense in which Descartes understood his first principle becomes clearer when we look at his answers to the objections made against it. On the one hand it was challenged by those who asked, like Gassendi, why the argument should be based especially on thought, and why we might not say with as good a right, _ambulo, ergo sum_: "I walk, therefore I am." Descartes explains that it is only as referred to consciousness that walking is an evidence of my existence; but if I say, "I am conscious of walking, therefore I exist," this is equivalent to saying, "I think in one particular way, therefore I exist." But it is not thinking in a particular way, but thinking in general that is coextensive with my existence. I am not always conscious of walking or of any other special state or object, but I am always conscious, for except in consciousness there is no ego or self, and where there is consciousness there is always an ego. "Do I then always think, even in sleep?" asks the objector; and Descartes exposes himself to the criticisms of Locke, by maintaining that it is impossible that there should ever be an interval in the activity of consciousness, and by insisting that as man is essentially a thinking substance, the child thinks, or is self-conscious, even in its mother's womb. The difficulty disappears when we observe that the question as to the conditions under which self-consciousness is developed in the individual human subject does not affect the nature of self-consciousness in itself or in its relation to knowledge. The force of Descartes's argument really lies in this, that the world as an intelligible world exists only for a conscious self, and that therefore the unity of thought and being in self-consciousness is presupposed in all knowledge. Of this self it is true to say that it exists only as it thinks, and that it thinks always. _Cogito, ergo sum_ is, as Descartes points out, not a syllogism, but the expression of an identity which is discerned by the simple intuition of the mind.[2] If it were otherwise, the major "_omne quod cogitat existit_" would require to have been known before the minor "_cogito_"; whereas on the contrary it is from the immediate consciousness of being as contained in self-consciousness that that major can alone be derived. Again, when Hobbes and others argued that thinking is or may be a property of a material substance, Descartes answers that the question whether the material and the thinking substance are one does not meet us at the outset, but can only be solved after we have considered what is involved in the conception of these different substances respectively.[3] In other words, to begin by treating thinking as a quality of a material substance, is to go outside of the intelligible world for an explanation of the intelligible world. It is to ask for something prior to that which is first in thought. If it be true that the consciousness of self is that from which we cannot abstract, that which is involved in the knowledge of anything, then to go beyond it and seek for a reason or explanation of it in anything else is to go beyond the beginning of knowledge; it is to ask for a knowledge before knowledge.