Ayn Rand is most well-known for her libertarian politics; she is also known for the individualistic aspects of her ethics. But she made various claims in epistemology; she has a theory, or a set of remarks, about the formation of concepts. The basic idea is that we are able to grasp, pre-conceptually, differences in degree of qualitative similarity and difference between objects, and form the ability to treat objects that are relatively qualitatively similar to one another as though they were — we are able to apply what we’ve learned from one object to another object in the future because the second object strikes us as similar to the first. She has what she thinks of as a technical innovation to explain how we do this; it isn’t successful except perhaps in a handful of cases. But here, I want to mention her description of the formation of concepts of mental states. She says that we form these concepts by recognizing the similarities between our own mental states, using introspective awareness as an analogue for perceptual awareness. But what kind of similarities?
In the realm of introspection, the concretes, the units which are integrated into a single concept, are the specific instances of a given psychological process. The measurable attributes of a psychological process are its object or content and its intensity. (ITOE 31)
It’s in terms of contents and intensity that mental states can be similar to, or different from, one another. What it is for a particular mental state to fall under the concept of a kind of mental state is for it to be, in terms of contents or intensity, commensurable with other mental states that fall under that concept: relatively similar. (Rand has trouble with the concept of commensurability; she thinks that being measurable by a common unit qualifies objects as similar to one another, when commensurability would only guarantee that it was similar to or different from it.)
On this account, I am the only thing in the universe that has any mental states. Take, for instance, the mental state of belief. Now, I have various beliefs. I believe that my backpack is blue and that this tea is lukewarm. These beliefs are beliefs because they are commensurable with other beliefs in terms of their content and intensity.
If you believe that this backpack is blue, or anything else, then my belief and yours are as commensurable, in terms of their contents, as any two of my beliefs. How we measure the facts or states of affairs or propositions that are the contents of beliefs, I can’t guess, but apparently Rand thinks that it’s possible.
But no belief of yours has any intensity comparable with the intensity of any belief of mine. There is no common metric; we can’t compare any belief of mine with any belief of yours according to their intensities. Thus my beliefs and the beliefs of another person are not commensurable. They are not similar (or even different) in intensity. Thus they do not fall under my concept of belief: they are not beliefs. No one but me has beliefs.
The same point would apply to concepts themselves: your concepts have no quantity of intensity that I can “omit” to bring them under my concept of concepts. Thus you have no concepts.
Thus, on Rand’s theory, I am the only being in the universe to have concepts, beliefs, or any other mental state at all. On Rand’s theory, Rand never accepted any theories.
Obviously, Rand did not accept solipsism (much less solipsism for me). But because of the casual and inattentive way in which she describes the workings of the mind, she did imply solipsism.