All quotes from Eef Dekker’s Was Arminius a Molinist? The Sixteenth Century Journal, Vol. 27, No. 2 (Summer, 1996), pp. 337-352.
Arminius: The knowledge of God is a faculty of his life, which is the first in nature and order, by which he distinctly understands each and every thing, whatever entity they have, will have, have had, can have, or might hypothetically have, and of each and every thing their order, connection, and various aspects that they have or can have; not even excluded that entity which belongs to reason, and which only in the mind, imagination or enunciation exists or can exist. (Public Disputation IV.30)
Dekker: … "Hypothetical entity" may sound just the same as "possible entity." There is, however, a weighty reason not to regard it as such. It is one of the characteristic features of Molinism to distinguish that which is possible from that which can hypothetically exist. In the first case it is about things that can exist, in the second it is about things that would exist, certain circumstances presupposed, as an effect of creaturely free will. In other words, the separation of categories (2) and (3) can be taken to be a first sign of Molinism.
Arminius: 2. [He knows] all possibilia, which may refer as it were to three genera: [a]The first is [knowledge of those things, to which the power of God may immediately extend itself, or which may exist by an act performed by him alone. [b]The second is [knowledge] of those things which, by God's conservation, motion, aid, concursuso, permission, can exist [as performed] by the creatures, whether these creatures will themselves exist or not, and whether they be placed in this or in that, or in infinitely many orders of things, [he knows] even those things which would exist by creatures, if this or that hypothesis were made. [c]The third is [knowledge] of those things which concerning the acts of creatures God can do-convenient for himself or for those acts. (Public Disputation IV.34)
Dekker: In category 2[b] formulations are used which bear close resemblance to those which Molina used in his Concordia The first formulation is "whether they be placed in this or in that, or in infinitely many orders of things." Molina formulates in his definition of scientia media: "were it [free choice] to be placed in this or in that or, indeed, in infinitely many orders of things.", The resemblance is striking, and I suggest that Arminius borrowed it from Molina. Still, it does not follow that Arminius has middle knowledge in mind. In fact, what Arminius says by the mouth of Molina has no connection with scientia media. Rather, it is about what creatures can do in specific situations, and not about the relation between a certain possible situation and human choice in such a situation. However, we actually do find middle knowledge in what follows on our quotation (2[b] at the end):"[He knows] even those things which would exist by creatures, if this or that hypothesis were made." Here again we have middle knowledge formulated.
Dekker: Middle knowledge, we now see, is right at the heart of Arminius' doctrine of divine knowledge. This conclusion is still reinforced by the fact that the traditional "middle knowledge" quotations from scripture, 1 Sam. 23:11-12 and Matt. 11:21, are not in the margin of thesis 43 (Public Disputation IV.43), but precisely next to our text: in the margin of category 2[b] of thesis 34 (Public Disputation IV.34).
Arminius: God's knowledge which ... is called "of simple intelligence" and natural or necessary is the cause of all things, by way of prescription and direction, to which is added the action of will and power, although it is necessary that middle knowledge intervenes in things which depend on freedom of created choice.(Public Disputation IV.45)
Dekker: …Natural knowledge apparently cannot prescribe to the divine will how to proceed in case of human free will. The divine will needs middle knowledge in order to know which free human act can be realized, given certain circumstances. It needs no further comment when I say that this rationale was precisely that which Molina had in mind when he "invented" middle knowledge.