It is a commonplace of the history of natural science and philosophy to think of change over time as a gradual clearing of the mist in all minds, as if each individual were gradually able to see better. However, the literature tells a different story; in every age we see traces of minds that were, with little evidence, clearly on the right track and in many cases had leapt with good reasoning to sound though very abstract conclusions. Epicurus is a good example, but every age is rich in such examples. 

If there is anything to the concept of the intellectual mist lifting, it is that more accurate concepts increase in frequency and may even come to dominate a population. In other words, that there is an average effect over the whole population of human minds. But this insight should remind us that this is not a monotonic phenomenon, with a burst of light – Let Newton Be... – after which irreversible progress has been made. Shifts in the population of mental representations are prone to ebb and flow, for a complex of reasons not all of which drive internal coherence and correspondence.

There is, of course, much evidence for the accumulation of vastly increased informational and evidential material over time. It is true that this too is prone to setbacks – Alexandrian libraries are sometimes burned – but the trend over the last two millenia has been towards increased volumes of evidence, and provided that the energy supply to human societies does not falter, and a sufficient part of that supply is devoted to informational maintenance, it is likely to continue.

But this simple increase in informational mass or volume is not a good description for change in intellectual history, where transitions are of a somewhat, though not entirely different character. Change is accounted for by fluctuations in predominance or frequency rather than appearances and disappearances; clear, correspondent statements may increase in predominance or frequency, rather than crystallizing out of fresh air. Of course concepts do, let there be no doubt about this, become more complex, detailed, fine-grained, qualified, and descriptively and predictively accurate, but the head terms that are the abstract summaries of a whole population of modern statements, change little, because they are sufficiently abstract to retain their purchase on life and matter. This accounts for the otherwise puzzling truth that while there is nothing new under the sun in science and philosophy it cannot be denied that real progress is made. Heraclitus prefigures thermodynamics, however eliptically; thermodynamics is not new, but it is genuine progress. All the sciences, without exception, provide similar examples. Intellectual progress is not a matter of sudden enlightenment, by transcendent revelation, but the refinement of propositions under existing headings, and an increase in the frequency of their representation in the population of minds.