It was this issue that Reid took seriously. He set himself, at first for his own satisfaction only, to consider the ground he had for trusting experience, when Hume reported that experience dissolved into sensational atoms. Accordingly he spent much of his time at New Machar in reflecting first of all on his perceptions through the senses. He began with these, because it seemed that even as the foundations of abstract mathematics lie in the mathematical axioms and definitions, so the foundations of all concrete reasoning are to be found in the rational constitution of perception through the five senses. ‘In the tree of human knowledge, perception is the root, common understanding is the trunk, and the sciences are the branches.’ Here is his own account of his state of mind when he was engaged in these New Machar wrestlings:— ‘If my mind is indeed what the _Treatise of Human Nature_ makes it, I find that I have been only in an enchanted castle, when I seemed to be living in a well-ordered universe. I have been imposed upon by spectres and apparitions. I blush inwardly to think how I have been deluded. I am ashamed of my frame, and can hardly forbear expostulating with my destiny. I see myself, and the whole frame of Nature, shrink into fleeting ideas, which, like Epicurus’s atoms, dance about in emptiness. Descartes no sooner began to dig in this mine than scepticism was ready to break in upon him. He did what he could to keep it out. Malebranche and Locke, who dug deeper, found the difficulty of keeping out the enemy still to increase; but they laboured honestly in the design. Then Berkeley, who carried on the work, despairing of securing all, bethought himself of an expedient. By giving up the material world, which he thought might be spared without loss, and even with advantage, he hoped by an impregnable position to secure the world of spirits. But, alas! the _Treatise of Human Nature_ wantonly sapped the foundation of this partition, and drowned all in one universal deluge.’ Nearly forty years after he left New Machar, Reid says[6] that in early life he ‘believed the whole of Berkeley’s system’—till Hume opened his eyes to ‘consequences’ that follow from the philosophy of Descartes and his successors, ‘which gave me more uneasiness than the want of a material world. So it came into my mind more than forty years ago’ to question its foundation. Hume accordingly made Reid revise critically the philosophy in which he had been educated by George Turnbull. The issue appeared after he left New Machar.