The following quote could be broken in two, as I think that each could stand alone. However, I'm taking directly from Lukacs's text. This is a part of a larger argument that he is making. In this mode, Lukacs is a thinker who takes flight high above the landscape, and therefore can move his eagle eye from one point to the next with little need for transition. Also, I might note that he begins by quoting the great William James (The best book on James, IMHO, is by Lukacs's friend, Jacques Barzun: A Stroll with William James.)
William James wrote: "You can give humanistic value to almost anything by teaching it historically. Geology, economics, mechanics, are humanities when taught by reference to the successive achievements of the geniuses to whom these sciences woe their being. Not taught thus, literature remains grammar, art a catalogue, history a list of dates, and natural science a sheet of formulas and weights and measures" [William James, Memories and Studies, 1911, pp. 312-313]
In sum, the history of anything amounts to that thinking itself. History is not a social science but an unavoidable form of thought. That "we live forward but we can only think backward" is true not only of the present (which is always a fleeting illusion) but of our entire view of the future: for even when we think of the future we do this by remembering it. But history cannot tell us anything about the future with certainty. Intelligent research, together with a measure of psychological understanding, may enable us to reconstruct something from the past; still it cannot help us predict the future. There are many reasons for this unpredictability (for believing Christians let me say that Providence is one); but another (God-ordained) element is that no two human beings have ever been the same. History is real; but it cannot be made to "work" because of its unpredictability.
At the End of an Age (2002), pp. 53-54.
I think another way to formulate Lukacs's insight comes from complexity theory: History (as the history of everything and everyone) is a very complex system in which sometimes seemingly trivial changes can have momentous effects. Society is not mechanical; it is complex. I think that Lukacs's interest in quantum physics should be updated by complexity theory. Thomas Homer-Dixon has written about his belief that ecology will replace physics as the master science. But in the end, the unpredictability of history (as the future) remains the same.