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I see three possible explanations of why philosophical metaphysics is never thoroughly commonsensical.
One is that a thoroughly commonsensical metaphysics wouldn’t sell. It would be too boring. A famous philosopher can’t say only obvious things. The problem with this explanation is that there should be at least a small market for a thoroughly commonsensical philosophy. Common sense might not be quite as fun as Nietzsche’s eternal recurrence or Leibniz’s windowless monads or Hegel’s world spirit, but a commonsensical metaphysics ought to serve at least as a foil; it oughtn’t be so downmarket as to be entirely invisible. In the 18th century, Thomas Reid helped found the Scottish school of “common sense” philosophy, and today he is the best known representative of that school — so one might naturally wonder if Reid’s metaphysics is thoroughly commonsensical. It’s not. See, for example,his thoughts on the immaterial sould of vegetables. Nor is G.E. Moore’s, when he develops his positive views in detail, despite his famous “Defence of Commmon Sense.”
Another possible explanation is that metaphysics is incredibly hard. There is a thoroughly commonsensical metaphysics out there to be had; we simply haven’t pieced it together yet. Maybe someday someone will finally bring it all together, top to bottom, with no serious violence to common sense at any point in the system. I fear this is wishful thinking against the evidence. (In a future post I hope to argue the point in more detail for the metaphysics of mind.)
A third explanation of the bizarreness of metaphysics is this: Common sense is incoherent in matters of metaphysics. Detailed examination of the consequences of our commonsense opinions inevitably leads to contradictions. To develop a coherent metaphysics in detail thus necessarily involves rejecting some aspects of common sense. Although ordinary common sense serves us fairly well in negotiating our everyday social and physical environments, it has not proven a reliable guide in cosmology or microphysics or neuroscience or evolutionary biology or probability theory or structural engineering or medicine or macroeconomics or topology. If metaphysics more closely resembles items in the second class than in the first, as it seems to, we might justifiably be suspicious of the dependability of common sense as a guide to metaphysics. Undependability does not imply incoherence, but it does seem a natural next step in this particular case, especially since it would generate a tidy explanation of the historical fact that detailed metaphysical systems are never thoroughly commonsensical.
Thus, I am endorsing the incoherence of common sense in matters metaphysical as an empirical hypothesis, justified as the best explanation of an empirically observed pattern in the history of philosophy.
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