Alfred North Whitehead, answered from the texts and cited to the page.
Eternal objects — what I sometimes call the Platonic forms — are the pure potentialities of the universe, the determinate possibilities that any actual occasion may realize in its becoming without themselves being actual occasions.1 They are not floating abstractions in some separate realm; the ontological principle forbids that. It is a contradiction in terms to assume that some explanatory fact can float into the actual world out of nonentity.2
Every potentiality must be somewhere, and that somewhere is an actual entity — specifically, the primordial nature of God, in whose conceptual valuation eternal objects constitute what I call the Platonic world of ideas.3 The key to understanding them is their dual reference. Considered publicly, an eternal object is a universal: in its own nature it refers to the general public facts of the world without disclosing the empirical details of its own implication in any particular actuality.4
Considered privately, it functions as the element that gives a subjective form its particular quality — the way a specific shade of colour or a specific emotional tone characterizes this occasion rather than that one. An eternal object can ingress into an actual entity in one of three ways: as an element in the definiteness of some objectified nexus that is the datum of a feeling, as an element in the subjective form of some feeling, or as an element in a conceptual or propositional feeling.5
What they are not is mere pendants to matters of fact. Possibility is definitely something — in some sense actual — not a mere pendant to matters of fact but in them.6 Eternal objects pervade all space-time, yet in any one occasion they are not simply impartially present; there is a selection, a preferential envisagement, through which this occasion takes account of certain potentialities and not others.7
That selection is what gives concrescence its direction, its subjective aim toward a determinate satisfaction. Without eternal objects there could be no novelty, no unrealized potentiality pressing toward realization; the alternative would be a static monistic universe in which the very word "potentiality" had lost its meaning.8
These potentialities are the 'eternal objects.' Apart from 'potentiality' and 'givenness,' there can be no nexus of actual things in process of supersession by novel actual things.Process and Reality, p. 72
It is a contradiction in terms to assume that some explanatory fact can float into the actual world out of nonentity. Nonentity is nothingness.Process and Reality, p. 72
The notion of 'subsistence' is merely the notion of how eternal objects can be components of the primordial nature of God... eternal objects, as in God's primordial nature, constitute the Platonic world of ideas.Process and Reality, p. 72
An eternal object considered in reference to the publicity of things is a 'universal'; namely, in its own nature it refers to the general public facts of the world without any disclosure of the empirical details of its own implication in them.Process and Reality, pp. 313–314
An eternal object can only function in the concrescence of an actual entity in one of three ways: (i) it can be an element in the definiteness of some objectified nexus... (ii) it can be an element in the definiteness of the subjective form of some feeling; or (iii) it can be an element in the datum of a conceptual, or propositional, feeling.Process and Reality, pp. 313–314
Possibility is definitely something - in some sense actual. - Not a mere pendant to matters of fact - but in them.Harvard Lectures 1925-1927, p. 372
'Eternal objects pervade all Space-time'. - In any one occasion not simply impartially there- but there is a selection of this taking account of.Harvard Lectures 1925-1927, p. 372
The alternative is a static monistic universe, without unrealized potentialities; since 'potentiality' is then a meaningless term.Process and Reality, p. 72