The retaining of beings for making present, as first encountered in the simile of the wax mass, is further elaborated through the simile of the aviary, as translated by Martin Heidegger in The Essence of Truth (215).
[Socrates] ‘Well, having and possessing seem different things. If a man buys a cloak and in this sense brings it under his power, but does not wear it, we should certainly say, not that he has it on, but that he possesses it’
[Theaetetus] ‘And rightly’
‘Now see whether it is possible in the same way for someone who possesses knowledge not to have it, in like manner, for instance, to a man who catches wild birds – pigeons or the like – and sets up an aviary at home in which to keep them. Might we assert that in a certain way he always has the birds, because he possesses them?’
‘And yet in another way he does not have any of the birds, but has acquired power over them, since he has brought them under his control within his own enclosure. He can take them and hold them whenever he likes, by catching whichsoever bird he pleases, and letting it go again; and he can do this as often as he likes.’
‘That is true’
‘And again, just as a while ago we contrived some sort of waxen figment in the soul, so now let us make in each soul an aviary stocked with all sorts of birds, some in flocks apart from the rest, others in small groups, and some solitary, flying hither and thither among all the others’
‘Consider it done. What next?’
‘We must assume that when we were children this receptacle was empty, and we must understand that the birds represent the varieties of knowledge. And whatsoever kind of knowledge a person acquires and shuts up within the enclosure, we must say that he has learned or discovered the thing of which this is the knowledge, and that precisely this is knowing’
‘So be it’
When we are born this container in our soul is empty, gradually it becomes filled with birds, i.e. we become familiar with beings and store them in the aviary. We have the birds stored there as possessions, they are left to their own devices. Thus we retain them but they are absent, but we can make them present by the action of grasping, taking up an active relationship towards them. Thus in the sphere of making present, memory, the same thing can be absent and present, we can have it as possession (ktesis) without expressly bringing it to mind. We can also select it by grasping it, taking an active relationship of being towards it (hexis).
Whoever possesses such an aviary possesses the doves in this cage, in this container, but does so in different ways. First, by sitting in a house, in a room, and having the doves under a roof. In this way, he can possess them and add to his possession. But he can also grasp a dove inside the container. There is the fundamental possibility of taking something out of this domain and having it in a stronger sense, taking on a relationship of Being with it. (Being and Truth, 198)