Draw a box. Inside the box, write “all the words in this box are false.” Is the statement true, or false? Now with that conundrum in mind, let’s proceed.
We expect someone to tell the truth because we generally expect (unless shown otherwise) that the person shares belief in the ethical or moral principle of truth. A principle such as “truth” is by its nature intangible, conceptual, eternal, and grounded in the mind, not the brain. We implicitly draw on the principle that truth is preferable to falsity, when we aspire to speak truthfully, and when we accord to someone else the presumption that they speak truthfully.
Does that presumption apply, when a materialist declares that materialism is true? Does it make sense in this situation to assume a shared principle that truth is preferable to falsehood?
Materialism is the belief that there is no supernatural reality; that the only reality available to us is physical. What that means is that a person is a body, only; that his thoughts are the result of a particular combination of physical conditions in the brain, and nothing more.
Because of this, materialists must hold to some form of determinism; that is, that a given action (say, my writing this, or your reading it) is the necessary result of all the past movements of matter in space and time. That actions at this moment are determined by the particular movement of every atom, from the beginning of existence down to the present instant. Obviously a particular action is influenced more by some vectors than by others. But the action is pre-determined by physical reality, materialists must hold.
To be sure, many materialists will differ with the notion of absolute determinism, and there are variations on the theme of determinism, philosophically. But ultimately, materialism begets determinism. This physical determinism would apply to everything, including what a person says (or writes) on a given occasion. So when a materialist says that materialism is true, why believe him?
The determinist response might be that we are evolved to be truthful, but that doesn’t work. Obviously people tell lies all the time, or shade the truth, or are mistaken. Moreover, evolution means self-selection for fitness. While fitness may be enhanced by telling truth on some occasions, it is enhanced on other occasions by telling lies, or by repeating mistaken beliefs as if they were true. At most, the determinist can posit a general tendency of human organisms to tell the truth because truthfulness is advantageous to the species. Not because truth is a worthy ideal in and of itself.
If our actions are determined by physics alone, there is no assurance of truth-telling on any particular occasion. The determinant for the content of speech is not truth, but the complex vectors comprising the speaker’s existence at the moment of the utterance.
As with truth in the abstract, so with rationality. It may be to evolutionary advantage to reason and to reason logically, but obviously that is a hit or miss proposition. The most dyed-in-the-wool materialist empiricist would agree that scientific observation and experimentation sometimes yields error, in which event the empirical process corrects itself. Indeed, that is a boast of those who rely on empiricism alone for truth.
What this means is that we cannot trust that a determinist has reasoned adequately, when he gets to the conclusion that there is no soul; no mind-body distinction; no supernatural; and no God. His conclusion, if true, would be just the product of all the physical variables that have gone before. Instead of rationality in the abstract, there would be only the individual’s physical mental processes on a particular occasion. To even discuss the soundness of one’s reasoning process, we have to invoke abstract ideals.
Materialists speak the language of truth and of rationality as ideals, even as their view of reality negates such ideals.