According to the Internet Encyclopedia of Philosophy,
Solipsism is…regarded as the doctrine that, in principle, “existence” means for me my existence and that of my mental states. Existence is everything that I experience — physical objects, other people, events and processes — anything that would commonly be regarded as a constituent of the space and time in which I coexist with others and is necessarily construed by me as part of the content of my consciousness. For the solipsist, it is not merely the case that he believes that his thoughts, experiences, and emotions are, as a matter of contingent fact, the only thoughts, experiences, and emotions. Rather, the solipsist can attach no meaning to the supposition that there could be thoughts, experiences, and emotions other than his own.
The problem of solipsism is one that is frequently raised in debates between Christians and atheists. Christians correctly point out that if there is no God, then there is no reason to suppose that my conscious experiences are not all that exist. I could be a brain in a vat, producing everything I perceive to be “external” to my person. This sometimes ends the debate. But sometimes the atheist responds by claiming that the Christian faces the same problem. And this makes the problem of solipsism one which we might want to be able to address, if pressed for an answer.
Oddly, many responses to the problem of solipsism, even among academics, amount to a kind of hand-waving dismissal, a philosophical “Are you kidding me?” response based upon “common perceptions of there existing a reality external to one’s own consciousness,” one which acts upon the senses quite apart from one’s active intellection. This response, however, isn’t strong enough. It’s similar to the scientistic atheists who dismiss the problem of induction by making an appeal to “the vast majority of cases” in which inductive reasoning has seemed to work. Ridiculing a philosophical problem is not the same thing as solving it.
Is there a better way to refute solipsism? Yes.
Whereas solipsism, on the one hand, denies that there is anything real “outside” of my consciousness, it also, on the other hand, affirms that I have knowledge. Put bluntly: This is a contradiction. Let me explain.
Firstly, knowledge is comprised of true propositions. What we know to be the case is what is actually the case. Oftentimes, someone will object by saying: “But I know several false propositions.” This objection hinges on a misunderstanding of the first assertion, viz. “Knowledge is comprised of true propositions.” When I say that knowledge is comprised of true propositions, I am stating that what we know is what is the case (e.g. A is A, A is not ~A, etc). When the objector hears this, he thinks I am saying that we do not have knowledge of the existence of a class of propositions whose logical value is false. That is not the case.
I know of the existence of two classes of propositions, (a.)true propositions and (b.)false propositions. It follows, then, that I know (a.)propositions state what is the case. I also know that (b.)propositions state what is not the case. When I say that our knowledge is comprised of true propositions, therefore, I mean that we know only what is the case. When it comes to propositions, it is the case that they are either (a.)stating what is the case or (b.)stating what is not the case.
So if I have any knowledge, I have knowledge of what is the case, what is true. And this is the problem for the solipsist, for if I can deduce what is the case of what I have not experienced, then what I have deduced already exists apart from my consciousness of it. This means that assertions which I have deduced from my present knowledge are not produced by my consciousness but apprehended by my consciousness. What may be validly deduced from my present knowledge, moreover, remains in existence even if I fail to deduce it, or even if I invalidly deduce some other conclusion. The proposition is present already outside of my own consciousness of it.
Yet if this is the case, then we have to ask: Where? Propositions are possessed by minds, not objects. Minds state what is or is not the case, not objects. Yet these propositions necessarily exist, for in order for me to be able to deduce them from my present knowledge, they must already be the possession of some mind other than my own. And if this is the case, then solipsism cannot be true. For if I experience coming to know the truth, then I am experiencing my acquisition of what has already existed prior to, and which will continue to exist despite my ignorance of, the truth.
So if I have any knowledge, I cannot be a solipsist.
What is striking about this is that it only get us as far as an implied mind outside of ourselves. The implication is a necessary one; however, since I do not experience that mind immediately, I am kept from saying that it exists solely within my consciousness. For if I fail to deduce that this mind exists outside of myself, it nevertheless follows necessarily that this mind does exist outside of myself. So my ignorance of the truth that “there is mind other than my own whose truth I have come to acquire” does not negate its reality.
So, again, if I have any knowledge, I cannot be a solipsist.
Knowledge is comprised of true propositions, and propositions are the possession of minds. But if I come to know a certain proposition is true by means of logical deduction, then I have found out what has already been true apart from my discovering it via deductive reasoning. And if I fail to come to know a validly deduced conclusion from my present, instead invalidly deducing a false conclusion from my present knowledge, I can only do so because there already exist validly deducible conclusions from my present knowledge, despite my not having consciousness of them.
Therefore, solipsism is false.
Soli Deo Gloria.