This relation may be one of contiguity, that is to say, the original experiences may have occurred at the same time or in close succession; or one of similarity (partial and not amounting to identity), as where the sight of one place or person recalls that of another place or person.

Finally, it is to be observed that recollection is often an act, in the full sense of that term, involving an effort of voluntary attention at the moment of revival.
Modern physiology has done much towards helping us to understand the nervous conditions of memory.

The biologist regards memory as a special phase of a universal property of organic structure, namely, modifiability by the exercise of function, or the survival after any particular kind of activity of a disposition to act again in that particular way.

The revival of a mental impression in the weaker form of an image is thus, on its physical side, due in part to this remaining functional disposition in the central nervous tracts concerned.

And so, while on the psychical or subjective side we are unable to find anything permanent in memory, on the physical or objective side we do find such a permanent substratum.
With respect to the special conditions of mnemonic revival at any time, physiology is less explicit.