Hume shows that causation can only ever be deemed a likely explanation for adjacent processes; it can never be proved.
It seems to us that striking a match causes a fire, because we know why it does and because the one effect follows the other action every time. Except it doesn't, of course, because it hasn't happened 'every time' yet. There remain an infinite number of times matches can be struck, and thus the ever-present possibility that our informed guess will be proved wrong.
But of course, Hume was not actually trying to disprove causation! Just as religious propagandists seize upon academic disputes within evolutionary theory to support the false claim that evolution itself is 'a theory in crisis', so advocates of paranormalism and exceptionalist phenomena cite Hume's dismantling of causation as support for a disordered, anything-goes universe.
One great philosophical service determinism performs is to put the rug back under causation: not only is the fire caused by the match striking but the match striking is caused by the decision to do so, and all of these things form part of a single network of causal processes stretching back to the beginning of existence.
Richard Dawkins was asked about determinism and free will and replied "I take refuge in complexity."
In other words, there has to come a point when causal mechanism becomes so complex that it becomes indistinguishable from discrete, spontaneously created, sequential occurrences and activity to even the most complex of observing intelligences, and at that point the debate becomes a meaningless one about terminology alone.
Positing a determinist view of mental existence and human agency need not demand any change in what we experience, merely in how we define it.
So in a profound sense, nothing has been taken away from existence.
What we are differing on is what we choose to call a phenomenon that we both experience in exactly the same way.
It is wrong, therefore, to say that purely mechanistic processes can account for 'the illusion of free will', because the word 'illusion' seems immediately to downgrade the quality, the 'reality' in a sense, of what is experienced as free will.
It is better to say that purely mechanistic processes are enough to account for free will exactly as you experience it.
Because that's all we really mean when we say 'free will': the sense of free will exactly as we experience it.
What practical difference is there between 'truly' free will and 'the illusion of' free will if the illusion of free will has all the characteristics by which 'truly' free will is defined? Indeed what, in such a context, can 'true' or 'illusion' possibly mean?
How is anything ultimately determined if not by the sum of its characteristics and effects?
Here we really have come across a major philosophical dispute that comes down ultimately to the use/misuse of language, and the imprecision of categories and concepts. For this is going further than merely saying, as Dawkins appears to mean, that there is no practical difference between unimaginably complex determinism and 'true' freedom of choice.
It is to say the two are literally the same thing. Both terms describe exactly the same impressions and experiences.
All that we are required to accept is that way, way back at the beginning of time, our mental experiences (in common with everything else in the universe) were caused.
If we believe otherwise - and assert, therefore, that we are different from everything else in the universe (and the implication is qualitatively rather than quantitatively so) - it is almost certainly because that's the way we want it to be.
This may be because it is the natural and instinctive way for any thinking entity to construct its sense of self.
Or it may be a rational conclusion, born of the fear that the alternative would rob us of dignity and meaning. This somewhat egocentric cosmology is usually supported with recourse to gods and divine sparks.
But how can determinism as I have formulated it possibly cheapen and degrade human experience?
I am not inviting you to some new future world where all our thoughts and actions are pre-determined by purely physical processes. I am saying they already are. I am saying: this is the world, the mental and the physical environment, in which you have always lived. So everything you experience in the future will be experienced just as it was in the past. Nothing will change.
You will feel the same, you will behave the same, and the sun will still rise in the morning because that's caused too.
It is the old fear of unweaving the rainbow, and this is the biggest, brightest, most sacred rainbow left in the sky.
But accepting determinism simply means acknowledging that the working of the brain, like the tide, is subject to chains of causation. This chain is so inconceivably long, so unimaginably complex, and composed of so many links that even vague comprehension of its pattern is literally impossible. No outcome can be predicted, no matter what position on the chain you start from.
This, surely, is to reconcile free will and determinism to the satisfaction of either's adherents. Far from incompatible, they are indistinguishable.
I say that human agency is determined by a mechanistic causality completely unknowable by human intellect, and that we have a name for this complete unknowability - we call it free will. It may not be 'true' freedom (whatever that means) but it's close enough to make the argument obsolete.