One of Collingwood's major views about history is that while the past no longer exists, it is possible by empathic projection from historical evidence or traces to understand why historical actors acted as they did. There is an unavoidable exercise of imagination but of imagination constrained by the evidence and by the need for coherent explanation. In other words, and for example, the evidence suggests that Gladstone converted to Irish Home Rule in the mid-1880s. There are records of speeches, private and public letters, memoirs of colleagues and so much else that make it reasonable to believe that Gladstone converted. But why ? What explains his change of mind ? The historian's task is to (try to) re-enact Gladstone's reasons : at the limit the propositional content of Gladstone's beliefs will coincide with the beliefs that on the basis of evidence and imagination the historian attributes to Gladstone.
Rex Martin puts the point in his own way :
Collingwood's conception of re-enactment furnishes the blueprint for
carrying through his ambitious program. According to Collingwood,
historical deeds, to be understood, have to be conceptualized as rational
solutions to situational problems facing historical actors. Historical re-enactment means explaining actions by going through the actual thought
processes of historical agents. For example, Hitler's puzzling decision to
invade the Soviet Union would no longer be perplexing were the historian able to show that, given the situation confronting Germany as
Hitler envisaged it, any rational person who had Hitler's goals and espoused his values, would have done the same. Hitler's decision will
remain inexplicable unless and until the historian can explain it as rational. Re-enactment, as Collingwood saw it, was a method peculiar to
history (and that part of philosophy which was historical) and served to
distinguish history from the sciences, both natural and social. The sci-entist pictures human behavior as events controlled by causes; the historian, as actions directed by thought. (Haskell Fain, 'Historical Explanation: Re-Enactment and Practical Inference by Rex Martin', The Journal of Interdisciplinary History, Vol. 9, No. 3 (Winter, 1979), pp. 525-527 : 525. Fain is here summarising his understanding of Martin's interpretation.)
I'm inclined to quibble over 'rational' : not only is this a many-ways ambiguous term but I think all the historian is required to do is to put herself in a position in which she understands - takes herself to understand - Hitler's beliefs and values well enough to see why, to Hitler, it made sense to invade the Soviet Union. (Parenthesis : I don't think many historians would describe Hitler's decision to invade as 'puzzling'; what perhaps is puzzling is his decision to declare war on the United States after the Japanese attack on Pearl Harbor.)
In light of this, we can see why for Collingwood historical knowledge is a matter of the historian's making sense for her- or himself of the actions of historical agents; and also why such knowledge, which is embodied in the activity of understanding, can't be learnt by heart or memorised. It has to be created by the historian. You can learn by heart or memorise what the historian tells you of the conclusions she has come to. But then you are not doing history - you are simply borrowing the results, collecting the sediment, of historical inquiry.
There are problems with re-enactment. One is that there is no objective criterion by which one can tell that one has re-enacted correctly the historical agent's reasons, whether Gladstone's or Hitler's. Another is that there are macro-phenomena such as the level of inflation or the behavior of a crowd for which re-enactment of individuals' thinking does not appear to provide an adequate explanation.
Collingwood, R.G., The Idea of History: With Lectures 1926-1928, ISBN 10: 0192853066 / ISBN 13: 9780192853066
Published by Oxford University Press, USA, 1994.
Ridley, Aaron, R G Collingwood (The Great Philosophers), 1998. ISBN 10: 0753805278 / ISBN 13: 9780753805275
Published by Phoenix.
Boucher, David (ed), The life and thought of R.G. Collingwood, ISBN 10: 0952439301 / ISBN 13: 9780952439301
Published by R.G.Collingwood Society, 1994.
Krausz, M., ed., Critical Essays on The Philosophy of R. G. Collingwood, Published by Clarendon Press, 1972.