Suppose (as happens often) that scientists, or philosophers, or explorers, or whoever, make some sort of surprising discovery, one that appears to be at odds with our commonsense view of the world. How should we react? It seems that there are three possible courses: either one rejects commonsense, or one rejects the alleged discovery, or one attempts to revise and/or reinterpret things to synthesize the two perspectives.
An example: periodically results come out in neuroscience which purport to show that some brain event, of which the subject is unconscious, occurs significantly before a subject makes a supposedly free conscious choice, and appears to deterministically cause that choice. This seems to conflict with the commonsense notion of 'free will.' A lot of people (especially the neuroscientists) seem inclined to jump to the conclusion of fatalism: commonsense is just mistaken; there's no such thing as free will at all. Others (especially philosophers) will point out that, even supposing there really is an instance of deterministic causation here, there are other aspects of the commonsense notion of free will which seem to suggest compatibilism. Since we really just can't give up the notion that we have free will, the thought goes, all the research shows is that when we reconstruct our notion of free will (which we need to do anyway, to remove the confusions, ambiguities, and perhaps even contradictions it contains), we should reconstruct it in a compatibilist vein.
Another example: it seems that some people, perhaps more in the past than today, have rejected the theory of evolution not primarily due to any religious doctrines, but just because they saw it as a shocking departure from the commonsense understanding of what it is to be human. ('Are you saying I'm just an ape with an overgrown brain?!') But of course the other two options are open here: we could say that commonsense is just wrong in supposing humans are special, or we could say that what is special about humans is consistent with them having common ancestry with other animals.
Now consider this: in the 18th century, it was widely thought that philosophers (Descartes, Locke, etc.) had made the surprising discovery that the only immediate objects of perception are 'ideas.' (What's an idea? Well, Hume clearly thought of them as sense data or copies of sense data, and I think Berkeley conceived of them basically this way as well. Descartes and Leibniz clearly thought that there were other ideas, the ideas of the 'pure intellect,' which were not in any sense images. Locke seems to me to be inconsistent on this point.) But, of course, commonsense holds that we immediately perceive tables and chairs and, perhaps more importantly, that we have immediate knowledge of tables and chairs. So what are we to do?
Well, we could just embrace philosophy at the expense of commonsense, as Hume did. Or we could embrace commonsense at the expense of philosophy, as Reid did. Berkeley, however, takes the reconciling strategy. At the end of Berkeley's Dialogues, Philonous summarizes the position as follows:
I do not pretend to be a setter-up of 'new notions'. My endeavours tend only to unite, and place in a clearer light, that truth which was before shared between the vulgar and the philosophers: the former being of opinion, that 'those things immediately perceived, are the real things'; and the latter, that 'the things immediately perceived, are ideas which exist only in the mind'. Which two notions, put together, do in substance of what I advance.
This framework, I believe, can help us to understand how Berkeley could, while advocating such apparently shocking claims, purport to be a defender of commonsense.
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