I was looking again at the Lewis paragraph which Alan cited - here it is again.
If (as many atheists claim) the world really boils down to a complex set of chemical and physical processes then love, beauty, meaning and even truth itself are merely illusions of a mechanical mind: “If minds are wholly dependent on brains, and brains on biochemistry, and biochemistry (in the long run) on the meaningless flux of the atoms, I cannot understand how the thought of those minds should have any more significance than the sound of the wind in the trees.”
Lewis isn't saying that brains can't generate experiences - which is something sometimes said by theists, along the lines that physical things can't produce mental things.
He is saying that these experiences are illlusions and meaningless. Actually, you don't need to mention love and beauty, since this argument would apply to any experience, e.g. the taste of chocolate.
I actually don't get this. Why is an experience an illusion, because it's produced by a physical organ (brain)? And why is it meaningless? If the brain generates the taste of chocolate, it's perfectly capable of generating its importance to me, whether big or small.
So I don't even get the force of Lewis's argument. The stuff about the flux of atoms is wacky, since nobody believes that my chocolate experience occurs when the atoms in my brain start tasting of chocolate, do they?