Editor’s note: In coming weeks, we will be featuring excerpts from the important new book Minding the Brain: Models of the Mind, Information, and Empirical Science (Discovery Institute Press, 2023). In this short excerpt, biologist Douglas Axe explains why common sense should never be divorced from the practice of philosophy (and science) and offers a defense of idealism as an alternative to both physicalism and substance dualism.

Over two centuries ago, Scottish philosopher Thomas Reid described the foundational role of common sense, and the folly of philosophers who think that they have somehow risen above it, as follows:

In this unequal contest betwixt Common Sense and Philosophy, the latter will always come off both with dishonour and loss; nor can she ever thrive till this rivalship is dropt, these encroachments given up, and a cordial friendship restored: for, in reality, Common Sense holds nothing of Philosophy, nor needs her aid. But on the other hand, Philosophy, (if I may be permitted to change the metaphor) has no other root but the principles of Common Sense; it grows out of them, and draws its nourishment from them: severed from this root, its honours wither, its sap is dried up, it dies and rots.

Based on the observation that each of us perceives the outer world from the perspective of our personal inner world, I have used the terms thinkers, thoughts, and things to denote categories that might reasonably be thought to encompass all of reality: “The outside world consists entirely of things (galaxies, atoms, trees, computers, our bodies, etc.), whereas each inside world consists of the mental space in which one thinker has thoughts.” The outside world, then, consists of everything that lies outside all inner worlds, making it equivalent to the physical universe, by this view.

 As commonsensical as this division of reality may be, there doesn’t seem to be a fully satisfactory way to make sense of it. Grouping thinkers together with thoughts as inside stuff implies that things stand alone on the outside — not just in the sense of being categorically different but in the more problematic sense of being irretrievably isolated. That is, if thinkers directly interact with thoughts only, such that those two categories occupy a realm completely separate from the realm of things, then what relevance could things possibly have to us thinkers? What basis could we have for believing they even exist?

 In preparation for showing how idealism provides a satisfactory solution to this puzzle, I will argue not only that physicalism comes up short but also that its best-known alternative — substance dualism — does too. Most strongly associated with René Descartes, substance dualism (henceforth, dualism) holds not just that the mind is distinct in substance from the brain but also that the possessor of mental states is “something quite over and above the states themselves, and is immaterial, as they are.” That is, dualists believe not merely that thoughts are immaterial but also that these exist only by being held in minds that are likewise immaterial.

I won’t suggest that dualism is as problematic as physicalism is. Instead, I see dualism as a good place for people to land when they recognize the untenability of physicalism, but idealism as the most compelling final destination for former dualists (of which I count myself).