De Moor IV:38: Middle Knowledge: Definitions and Clarification of the Question
Moving beyond words to the substance of the Doctrine, it is a Knowledge that is called Middle or Conditioned, whereby God knows Free and Contingent Future Things under Condition prior to the Decree.
It is called Middle, because it as Middle comes between the Knowledge of Simple Intelligence and Free Knowledge, and it is, as it were, of a mixed condition. It agrees with Natural Knowledge in this, that it precedes the determination of the divine Will decreeing concerning this matter: it agrees with Free Knowledge in this, that it concerns Future Things; and that, just God by Free Knowledge was able not to know future things, if He had decided not to produce them, so also He by Middle Knowledge was able not to foreknow those things that creature were actually going to do, if according to their liberty they were not going to do those things, but something else. At the same time, Middle Knowledge differs from the two other kinds previously mentioned: from Natural Knowledge, because it concerns Future Things, but not Possible Things; but from Free Knowledge, because it is not of things certainly future, but only hypothetically. It is also able to be called Middle, because it as a Middle comes between the Orthodox opinion concerning the Knowledge of God, and the Socinian opinion, which altogether removes from God the Foreknowledge of Contingent and Free Future Things.
It is also called Conditioned, because the Outcomes of events foreseen by this Knowledge are dependent upon a certain condition, and under that condition are thought to be known by God.
And so this Middle Knowledge concerns the Free Actions of Rational Creatures, inasmuch as what they, if they be placed in these or those circumstances, are going to do according to the free inclination of their will, God foresees, and that before any particular Decree of His Will concerning this matter.
It is not asked simply, whether God knows Free and Contingent Future Things, which we certain acknowledge; but whether for that is require a third sort of Knowledge, Middle Knowledge, distinct from Natural and Free Knowledge, which we deny.
The question does not concern Future Conditioned Necessities, which with the condition posited are not able not to come to pass, inasmuch as these entail a necessary connection between the antecedent and the consequent, either from the nature of the thing, or upon hypothesis of the divine Decree, which sort of things are, for example, if the sun rises, then the day dawns; if Peter repents from the heart, he will be saved: for these things fall under the Knowledge of God, either Free, if the condition be future and decreed by God; or Natural, if the condition is merely possible. But the question concerns Future Conditioned Contingencies, which with the condition posited are able to be and not to be, for example, if Peter were at Jerusalem, he would speak, sin, etc.: are these things certainly and definitely known by God antecedently to the Decree through Middle Knowledge? which we deny.
We do not give attention to any general Decree, whereby God will have decreed to produce second causes, and whereby He prepared to furnish a general and indifferent Concursus to the creature, since He willed that they determine their own actions, which our Adversaries grant to precede this Knowledge; while at the same time they deny a special Decree concerning the certain futurition of this or that thing, preceding this Knowledge.