 In discussions of early modern philosophy in the second half of the twentieth century, Descartes, Locke, Berkeley, and Hume were sometimes portrayed as sense-data  theorists,[5]  especially when suggesting that they abetted skepticism about the external world. Indeed, the Scottish philosopher Thomas Reid diagnosed the external-world skepticism found in Hume as arising from the theory of ideas (initiated by Descartes and adopted by the others) being taken by Hume to what Reid considered to be its logical conclusion (Nichols &amp; Yaffe 2014 [2016]). This interpretation was known to our early sense-data theorists through study of Reid himself or via William Hamilton’s Lectures on Metaphysics (1861), in which Reid’s claims were discussed. In the latter half of the twentieth century, Richard Rorty (1979: ch. 3) portrayed Descartes and Locke as engendering a veil of perception. He did not use the term “sense data” to describe the fabric of the veil by which these authors isolated perceivers, but he endorsed Reid’s charge that they created a veil of ideas. This reading has been challenged (e.g., in Yolton 1984), and the present entry qualifies it in many respects.