99. Dugald Stewart has justly dwelt on the signal service rendered by Descartes to psychological philosophy, by turning the mental vision inward upon itself, and accustoming us to watch the operations of our intellect, which, though employed upon ideas obtained through the senses, are as distinguishable from them as the workman from his work. He has given indeed to Descartes a very proud title, Father of the experimental philosophy of the human mind, as if he were to man what Bacon was to nature.[244] By patient observation of what passed within him, by holding his soul as it were an object in a microscope, which is the only process of a good metaphysician, he became habituated to throw away those integuments of sense which hide us from ourselves. Stewart has censured him for the paradox, as he calls it, that the _essence_ of mind consists in thinking, and that of matter in extension. That the act of thinking is as inseparable from the mind as extension is from matter, cannot indeed be proved; since, as our thoughts are successive, it is not inconceivable that there may be intervals of duration between them; but it can hardly be reckoned a paradox. But whoever should be led by the word essence to suppose that Descartes confounded the percipient thinking substance, the Ego, upon whose bosom, like that of the ocean, the waves of perception are raised by every breeze of sense, with the perception itself, or even, what is scarcely more tenable, with the reflective action, or thought; that he anticipated this strange paradox of Hume in his earliest work, from which he silently withdrew in his Essays, would not only do great injustice to one of the acutest understandings that ever came to the subject, but overlook several clear assertions of the distinction, especially in his answer to Hobbes. “The thought,” he says, “differs from that which thinks, as the mode from the substance.”[245] And Stewart has in his earliest work justly corrected Reid in this point as to the Cartesian doctrine.[246]