 In addition to this, Locke calls the substantial nature of souls into question. Locke takes thought to be immaterial, and while Locke contends that the immaterial cannot be reduced to, or explained in terms of, the material, Locke is not committed to substance dualism, when it comes to finite thinkers. This is because Locke thinks substratum—or the substance that underlies and supports any particular substance’s qualities—is impossible for finite minds to penetrate. Additionally there is nothing in the concepts “thought” and “matter” that allows us to deduce that one excludes the other, and God could have superadded the ability to think to formerly inert systems of matter. In Locke’s picture, we cannot know whether the substance (or substratum) that underlies thinking and willing is different from the substance (or substratum) that underlies being solid and white, or yellow and malleable. Locke’s official position on the substantial nature of finite thinkers is therefore one of agnosticism.