Both determinacy and freedom are necessary aspects of temporal reality. And, so, because we are naturally and ineluctably temporal creatures, both determinism and indeterminism are true for us: but this, in different ways, for they pertain to different temporal epochs.
Determinacy pertains to the past of every occasion, and indeterminacy to its present.
The past must be determinate, or it would not be anything definite at all, and thus have no actuality. As inactual, it could have no effect on any futurity, and could not play any causal role in any world.
The past is actual, and fully definite, so it is fully determined: it has no option to be other than what it is. And as we look back upon it – this being the only way that temporal events can possibly look (for, of what is not yet fully and definitely actual, there is nothing yet definite to be seen) – we can – in principle, at least – see how each past act is completely, perfectly, and indeed beautifully coherent with its causal antecedents, so that between it and those antecedents there are no causal gaps, no inconsistencies, nothing left out of the causal account, and no loose causal ends. Only thus, looking back, could we apprehend the world as a causal system consistent with itself, and orderly; as, i.e., a world, properly so called: a seamless web. Then we can see looking back, in other words, that past events are, from our present perspective as an occasion of lively becoming, all fully determinate.
Determinism is true of the past of each moment of becoming.
The present moment of becoming is not in every respect determined by the past, or by what is factual – including eternal facts. If our present moments of becoming were thus determined, they could have no effects of their own upon futurities, and would not therefore act in any way: they would not be actual. They could not then play any causal role in any world.
Indeterminism is true of each present moment in itself, and for itself, as it comes to be actual, definite, and determined; as it determines itself. The moment of becoming, then – which we experience as the present – is what it is like to act. The present is the realm of indeterminacy, and thus of freedom.
Occasions of becoming are each of course very greatly constrained by their actual worlds – by their past, which is their causal environment. That constraint is no tyranny, but rather an information, indeed a sort of education. It is the indispensable theater of free becoming, the ordered environment in which alone orderly action can proceed in the first place. For, it is not possible to behave either properly or improperly, either well or ill, in respect to chaos. Indeed, in respect to chaos it is not possible to act at all, period full stop. Nothing appertains to chaos; to nothing, nothing only appertains.
The project of coming to be an actual event involves finding a means of agreement more or less happy – and in all ways perfectly coherent with – the environing past. It involves finding the best way to be that is compossible with that past. Each such discovery involves a search for optimality in what Borges called the Library of the Possible, among the various options in that Library that are compossible with its past. Each such option will be an absolute novelty: the realm of those options is the future of each occasion, which exists in respect to it only virtually.
Those options are constrained and ordered ex ante and logically – are, i.e., limited – by the logical order and character of the environing actual past. The logical structure of its past limits what is possible to each new instant. Viz., it is not possible given the logical character of acorns and carburetors – given, that is to say, their essential natures – for an acorn to give rise to a carburetor, or vice versa. Its logical character does not, however, prevent an acorn from giving rise to countless different possible oak trees, each with an unique career.
The logical character of things, and their resultant logical and causal relations in every possible world line of every possible cosmos, are of course given necessarily in the eternal Lógos.
To become is to intend some future option, not just for oneself, but for the whole world. To choose to be x is to suggest to all subsequent occasions that the world ought to have x in it; would do well to have x in it.
The past then, which is entirely composed of actual events that each intended and proposed to its subsequents a particular future, taken together intends and proposes to us a certain sort of future. Such is its aesthetic appeal. No such appeal, then no suasive effect of a preterity upon its futurities.
Determinism is completely true, then, but only of past and fully actual events. And indeterminism too is completely true, but only of events presently becoming.
Post Scriptum: becoming that is causally tied to a past of some sort is essential to time; and so is that becoming; and because an event now transpiring cannot act, and thus be actual, except in respect to futurities, so temporal becoming as such – and, so, time: any world that extends temporally – necessitates futurities. A sequence of events cannot be obtained without both a fixed past and an indeterminate present that in its due time will add to that past, thus generating a newly modified past as the causal input of futurities. We could not have time – past, present, or future – without all three causal epochs. So, both determinacy and indeterminacy are indispensable to worlds such as ours.
The angelic worlds are of course a different story; likewise, the demonic. This post is not about them.