Consider this sentence: Zebras are heavier spellers than giraffes.
The sentence is easily seen to be incoherent. Even though the words all have meaning, and the structure or syntax of the sentence is correct, the words don't fit together in a meaningful way. The proposition is literally nonsense.
Now consider this state of affairs: a colleague walks into the office and reports that it has begun to snow. You look out the window and confirm that it is indeed snowing. Your observation corresponds to your colleague's report.
In both cases something very interesting is going on, usually without us being aware of it. Somehow, we're able to recognize phenomena like incoherence and correspondence, but how? What is actually occurring in our cognitive apparatus when we recognize that words don't cohere or that other words accurately describe what's happening outside?
On the materialist view all that's involved here are electrochemical processes occuring in a lump of living matter in our heads, but how do events like electrons whizzing along neurons and molecules jumping across synapses cause us to recognize incoherence and correspondence?
What's the connection between the physical processes taking place in the brain when we read the sentence or look out the window and the conscious awareness that a particular pattern of words is either absurd or does in fact correspond to an actual state of affairs? Where in the brain does the awareness of incoherence and correspondence reside and what does this awareness "look" like?
Moreover, what would a tiny, miniaturized scientist, navigating her way through a brain which at that moment is recognizing incoherence, observe? She might witness a great deal of electrical activity and notice a lot of atoms jostling about, but how do these physical phenomena translate into an immaterial awareness that a combination of words is nonsensical?
Materialists will often reply that it's true that at this point we have no idea, but that we know too little at present about how the brain works to say how it does what it does. Someday, though, we'll able to explain it, and when we do the explanation will be in completely material terms.
Well, maybe, but there's a pretty serious problem here.
Physical things have weight, occupy space, and are made of atomic particles that possess electric charges. Mental events, on the other hand, share none of these properties. They have no weight or mass, they're non-spatial, they're made of no substance recognizable to science and they don't possess electric charge.
All that being so, it's clear that mental events such as recognizing incoherence and correspondence are not themselves material or physical. They're qualitatively different, so, in the absence of any plausible materialistic explanation, it makes sense to suspect that perhaps these mental events are somehow the product of an immaterial, non-physical element in our cognitive apparatus.
In other words, perhaps the physical brain is not the sole agent in our cognitive life. Perhaps the conscious experience of human beings, and maybe even other animals as well, is also the product of a spaceless, massless, immaterial mind.