certain as that of body, even though the one substance is as little known as the other, he asks (pp. 241 ff) how reflection can assure us of the existence of mind if God can, as our author claims (Essay iv.iii), give matter the faculty ÑÑof thought; since then the way of ideas, which should distinguish what is proper to the soul as opposed to the body, would become useless; whereas it was said in the Essay 11.xxiii.15, 27, 28 that the operations of the soul provide us with the idea of mind, and that the understanding and the will together make this idea as intelligible to us as solidity and impulse make the nature of body. In his first letter, our author gives the following reply (pp. 6sfF): I believe that I have proved that there is a spiritual substance in this, for 'we experiment in ourselves thinking.1 This action or mode cannot be the object of the idea of a self-subsistent thing and therefore this mode requires Ôa support or subject of inhesion[, and] the idea of that support is what we call substance.... The general idea of substance being the same every where, it follows that when the modification which is called thought or power of thinking is joined to it, that 'makes it a spirit, without considering what other modifications it has, [i.e.] whether it has solidity or no. As on the other side, substance, that has the modification [which is called] solidity, is matter, whether it hasÕ thought or not. But if by spiritual substance you mean Ô an immaterial substance, I grant I have not proved, nor upon my principles can it be proved' demonstratively that there is such within us. But what I have said about the systems of matter (Essay iv.x.16) in demonstrating that God is immaterial makes it 'in the 63 highest degree probable, that the thinking substance in us is immaterial. Õ ...However, I have shown (the author adds on p. 68) that Ô the great ends of religion and morality are secured.. .by the immortality of the soul, without a necessary supposition that the soul is immaterial'. The learned Bishop, in his answer to this letter, in order to show that our author held a different view when he wrote the second book of the Essay, quotes (p. 51) the following passage from it: ÔBy the simple ideas we have taken from those operations of our own minds... we