Open theism means more than one thing – I here am thinking of the open theism that argues that God does not know the future because there is no such thing to know. Ruyer argues something similar, sc. when we get in a car accident, we run into another car, not into the accident. There is no “future accident”, waiting like a ghost out there for us to run into it from behind, and therefore no “future accident” to be gazed at, whether by God or anyone else.
This is one of the fullest accounts STA gives of the mode of existence of the future, and so the strongest and most charitable critique of his ideas would probably start here. A fuller translation would have used the phrase “under the ratio of”, here usually just translated “as”.
The future can be known in two ways 1.) in itself and 2.) In its causes.
1.) In itself it can be known by no one but God. The reason is that future things, as future, do not have existence in themselves; and since existence and truth are convertible and all knowledge is of something true, it is impossible that there be some knowledge of the future as future. But since the past, present, and future are the differences of time and designate temporal order, everything that is in time stands to the future as future. So it is impossible that a knowledge in the temporal order would know the future in itself, and the knowledge of every creature is such (cf. infra) so it is impossible for some creature to know the future in itself. This is peculiar to God alone, whose knowledge is above the whole order of time, so that no part of time stands to the divine operation as past or future, but the whole flow of time and the things that are done in the whole of time fall under his gaze and are conformed to it in the mode of the present. His undivided act of vision bears down on all things simultaneously, as everything in its own time. A likeness can be taken from the spacial order. Just as the prior and posterior in motion and in time follow the prior and posterior in magnitude, so God in the mode of the present sees all things which are compared to each other in order of past, present and future, which cannot happen for one whose vision falls under the order of time, just as someone situated in a high vantage point sees all walking on the road, not as coming before and after him although he sees others so proceed, but everyone situated on the road can see nothing but what preceded him or what is positioned next to him.
De malo, 16. 7.
[D]upliciter possunt futura cognosci: uno modo in seipsis, alio modo in suis causis. In seipsis quidem a nullo cognosci possunt nisi a Deo. Cuius ratio est, quia futura, prout futura sunt, nondum habent esse in seipsis; esse autem et verum convertuntur; unde cum omnis cognitio sit alicuius veri, impossibile est quod aliqua cognitio respiciens futura in ratione futuri, cognoscat ea in seipsis. Cum autem praesens, praeteritum et futurum sint differentiae temporis, temporalem ordinem designantes: omne quod qualitercumque est in tempore, comparatur ad futura sub ratione futuri. Et ideo impossibile est quod aliqua cognitio subiacens ordini temporis, cognoscat futura in seipsis. Talis autem est omnis cognitio creaturae, ut post dicetur. Unde impossibile est quod aliqua creatura cognoscat futura in seipsis; sed hoc est proprium solius Dei, cuius cognitio est elevata supra totum ordinem temporis, ita quod nulla pars temporis comparatur ad operationem divinam sub ratione praeteriti vel futuri; sed totus decursus temporis, et ea quae per totum tempus aguntur, praesentialiter et conformiter eius aspectui subduntur. Et eius simplex intuitus super omnia simul fertur, prout unumquodque est in suo tempore. Potest autem accipi conveniens similitudo ex ordine locali. Sicut enim prius et posterius in motu et tempore consequitur prius et posterius in magnitudine, ut dicitur in IV Physic., ita Deus praesentialiter omnia intuetur, quae ad invicem comparantur secundum ordinem praesentis, praeteriti et futuri. Quod non potest aliquis eorum cuius intuitus sub hoc ordine temporis cadit; sicut ille qui est in alta specula constitutus, videt simul omnes transeuntes per viam, non sub ratione praecedentis et subsequentis quoad ipsum, quamvis videat quosdam alios praecedere; tamen quicumque in ipsa via constitutus est in ordine transeuntium non potest videre nisi praecedentes, vel iuxta se positos.