Here are excerpts from Étienne Gilson’s Methodical Realism (Le réalisme méthodique), translated by Philip Trower (Christendom Press, 1990 / Ignatius Press, 2011):
The mathematician always proceeds from thought to being or things. Consequently, critical idealism was born the day Descartes decided that the mathematical method must henceforth be the method for metaphysics. p.11
Indeed, all idealism derives from Descartes, or from Kant, or from both together, and whatever other distinguishing features a system may have, it is idealist to the extent that, either in itself, or as far as we are concerned, it makes knowing the condition of being. p.12
With Descartes the Cogito ergo sum [I think, therefore I am] turns into Cogito ergo res sunt [I think, therefore things are]. p.13
A thought which starts from a mental representation will never get beyond it … p.14
Realism may possibly be a neo-scholastic doctrine. But it is definitely not a scholastic one. p.17
As used today, the word realism means in the first place the opposite to idealism when it claims that it is possible to pass from the subject to the object. Applied to medieval metaphysics it means a doctrine in which the real existence of the object is taken for granted .. p.18
Reality can be grasped at levels of different depths. It is immediately given to us in a kind of block form, which is simply the “apprehended reality”. p.49
“The scholastics”, said Spinoza, “start from things; Descartes from thought; I start from God.”
He could not have said anything more true, and the name of Spinoza is enough to remind us why in fact the scholastics do not start from God.
Between the Christian God and things there is a metaphysical fissure, separating the necessary from the contingent. The world only exists by a free ordinance of God; consequently, it cannot be deduced from God. p.52
While Descartes finds being in thought, Saint Thomas [Aquinas] finds thought in being. p.53
Up to Descartes’ time, and particularly during the Middle Ages, it had always been agreed that philosophy consisted in a transposition of reality into conceptual terms. p.62
By turning reality into a mosaic of clear ideas, Cartesian mathematicism raised difficulties which the whole of the seventeenth and eighteenth centuries tried to resolve, and which the nineteenth century finally despaired of resolving, at the same time despairing of philosophy itself. p.64
Having left us with thought (not a soul), and extension (not a body), [Descartes] does not know how to account for the union of soul and body. p.64
Having expelled quality from the field of extension, [idealists] do not know how to account for it when it reappears in thought. p.64
Hume’s skepticism, therefore, descends in a direct line from Cartesian mathematicism. p.66
If there is a single initial error at the root of all the difficulties philosophy is involved in, it can only be the one Descartes committed when he decreed, a priori, that the method of one of the sciences of reality was valid for the whole of reality. p.69
The scientific sterility of the Middle Ages has to be condemned for the same reasons which make it necessary today to condemn the philosophic sterility of scientism. Aristotle, too, exaggerated the scope of a particular science and the value of its method to the detriment of others. p.72
Aristotle’s error was philosophically less dangerous because it was an error of fact and left the rights of philosophy intact. In biologizing inorganic nature, Aristotle and the medieval philosophers condemned themselves to ignorance about the sciences of the inorganic world … p.72
An Aristotelian discourse on the method is therefore an impossibility. One can speak, in the Aristotelian context, only of a discourse on methods. p.73
If there is something more in a living being than a pure mechanism, Descartes is bound in advance to miss it. p.74
So our first duty today is to be more faithful to the demands of realism than the Middles Ages were, and giving each order of reality its due. p.75
Between Cartesian “artificialism”, which turned animals into machines, and Aristotelian vitalism, which treated physical bodies as if they were animals, it should be possible to find room for mechanism in the physical order and vitalism in the biological. Every “nature” requires a formal principle, but not every form is a living form. p.76
Most of our contemporaries think that, at bottom, being a philosopher and adopting an idealist method are one and the same thing. p.79
Essentially, [philosophy] consists in a considered choice between two possible methods, Aristotle’s and Descartes’. Either one begins with being, in which thought is included ab esse ad nosse valet consequentia [from a thing’s reality one can be certain of its possibility], or one starts from thought, in which being is included a posse ad esse valet consequentia [from its possibility one cannot be certain of its reality]. p.84
I maintain, therefore, that just as there is in Cartesianism a methodical idealism, the kind that starts with nosse [knowing], there can be a methodical realism, the kind that starts with esse [being]. p.85
Every given reality implies the thought which apprehends it. Therefore being is the condition of knowing; knowing is not the condition of being. p.86
Realism does not reject the idea of a critique of the different kinds of knowledge. It accepts it; it calls for it. But it does reject all a priori critique of knowledge as such. Instead of prescribing limits to reason a prior, which soon become limits to reality itself, realism accepts reality in toto and measures our knowledge by the rule of reality. p.87
If the being, insofar as it can be conceived, is the first object of the intellect, that is because it is directly perceived: res sunt, ergo cogito [things are, therefore I think]. We start by perceiving an existence which is given us in itself and not first of all in relation to ourselves. p.87-88
The realist method starts with the whole in order to distinguish the parts. p.88
All realism derives from the analysis of knowledge; all idealism derives from the analysis of a thought. p.89
Once the true is no longer being as known by the mind, what is truth? Once the good is no longer being as the object of desire, what is the good? And, consequently, what are science and morality? It was at this point that the good, the true, and the beautiful began to transform themselves into values, because values are simply transcendentals [properties of being] that strive to subsist after they have severed their connection with being. p.89
… we can only re-establish metaphysics today by returning to realism pure and simple. p.92
For Chapter V, see here.