The question of whether I could really ever know if the people I knew and loved were essentially the same as me, whether they experienced the world similarly to me, whether they were conscious, is an epistemological question. Epistemology is the philosophical study of knowledge. Epistemological solipsism is the stance that the only things that can be known are one’s own thoughts and experiences. I can only know with certainty what is in my own head. The existence of other minds could be possible, but I can never know with absolute certainty. The philosopher Rene Descartes built his epistemic theory on the foundation, “Cogito ergo sum,” or “I think, therefore I am,” or “I think, therefore I exist.” That is to say, if he doubted everything he thought he knew, the only thing that he couldn’t doubt was the existence of his own self.
“But I have convinced myself that there is absolutely nothing in the world, no sky, no earth, no minds, no bodies. Does it now follow that I, too, do not exist? No. If I convinced myself of something [or thought anything at all], then I certainly existed. But there is a deceiver of supreme power and cunning who deliberately and constantly deceives me. In that case, I, too, undoubtedly exist, if he deceives me; and let him deceive me as much as he can, he will never bring it about that I am nothing, so long as I think that I am something. So, after considering everything very thoroughly, I must finally conclude that the proposition, I am, I exist, is necessarily true whenever it is put forward by me or conceived in my mind.”1
From this basic foundation, Descartes inferred the existence of the material world and God. Although his Meditations on First Philosophy was a groundbreaking philosophical work at the time it was written, it is now generally thought by philosophers today that Descartes made some unsound reasoning in inferring the existence of the material world and God from the foundation, “I think, I exist.” However, this foundation is probably the most striking example of epistemological foundationalism in the history of philosophy (to my belief). (Foundationalism is a theory of justification for knowledge. For a discussion of foundationalism v. coherentism—another theory of justification—I advise you to consult the Stanford Encyclopedia of Philosophy on the Internet.)

The question of whether or not my mind was the only thing that really existed, whether others were just a product of my mind, is a matter of metaphysical solipsism. Metaphysics is the philosophical study of reality, or that which really exists. In metaphysical solipsism, the self is the only thing that exists. As a three year old I quickly dispelled my thoughts of metaphysical solipsism in my own inexperienced way. I simply loved my family too much to really want to believe that I was all alone in reality, but the doctrine cannot be so hastily dealt with if one is to give it the serious philosophical thought it deserves. With some thought, the question soon arises: How do I account for the unpredictable behavior and the vast unknown in the world if it all arises from my own mind? Certainly I couldn’t ever have anything to learn if everything in my sense experience was a product of my own mind. Why am I so often surprised by what I experience? But these questions aren’t enough to infer the existence of other minds like mine to explain people’s unpredictable behavior. There is another possibility. Perhaps God is creating these experiences for me, and I am God’s only creation. This would explain the unpredictability of the “world” around me, but there is another possibility as well: that the unpredictability in the “world” arises from a part of my mind of which I am not consciously aware, perhaps my subconscious. These may seem like odd questions to entertain, but if we are to practice philosophy seriously, we can’t just dismiss them as sounding odd. We have to have good reasons to believe what we believe.

Metaphysical solipsism arises from a stubborn adherence to epistemological solipsism. Certainty need not be the foundation of knowledge. In fact, most people would probably agree that knowledge would be very limited if it depended on certainty (if most people even think about such things). For example, I may not be able to honestly say with certainty that I watched Countdown today, even though I believe I did. There is a possibility (however remote) that I dreamt doing that. To be sure, knowledge in the study of philosophy these days can be loosely defined as ‘justified true belief’. As long as you hold a belief that is justified and true, then it may be considered knowledge. So, based on the coherence and consistency of my belief that I watched Countdown today with my other beliefs, viz. that I remember waking up this morning, that I remember turning to channel 358 at 7:00 PM, that I learned things on the program that were corroborated on other current event programs I watched today, that stories I watched on the program were also mentioned in articles online, etc; then I can be confident that my belief is justified. If this belief happens to be true (if it corresponds to actual states of affairs in the actual world) and I honestly hold it, then it is knowledge. (However, it has been suggested by some philosophers that there is another component to knowledge besides justification, truth, and holding the belief. Without getting into a lengthy discussion on the theory of knowledge, however, we’ll have to leave it at that. After all, we are talking about solipsism.)

Likewise, since I have good evidence that other people have brains, and similarly I have a brain, and because of the other anatomical equivalencies between me and other human beings, and because of their similar behavior, I can infer inductively by analogy that based on this evidence, they are conscious like me. This is the most plausible possibility, and I think most ‘normal’ people would agree. (Yes, even most of those peculiar philosopher types probably agree.) This would explain the unpredictability in others’ behavior without having to hold the peculiar and rather unlikely belief that I am God’s only creation or that people are a product of my subconscious. (Although for reasons I am not going to get into right now some philosophers argue that inductive reasoning is not a valid form of logic, the majority of philosophers hold that inductive reasoning is a valid form of logic that is necessary for any kind of real knowledge about the world.) I will even venture to say that these beliefs—my having a brain, others having brains, my behavioral and anatomical similarities with other people—are justified because they are coherent and consistent with my experience and between themselves as beliefs. From these justified beliefs I can infer inductively (and strongly I venture to say) that other people are conscious beings like me. This inference is justified by its consistency and coherence with my other beliefs and my experience. I do hold the belief that others are conscious like me, so if it happens to be true (if it corresponds to actual states of affairs in the actual world), then it can be safely asserted that I know others are conscious like me (or you might say “audaciously asserted” if you are rather odd). So much for solipsism.