One can distinguish during the modern period preceding Kant two antagonistic forms of idealism: a rationalist idealism and an empiricist idealism. The first, oriented towards the concepts of reason, developed essentially between the middle of the 17th century, after René Descartes, and the beginning of the 18th century, with Gottfried Leibniz. The second, which relies on the senses, is defended mainly by George Berkeley during the first half of the 18th century, although John Locke can be considered its precursor.
Descartes
(Diagram of vision from a drawing by René Descartes, which illustrates the strictly mechanical character of perception before the stage of the production of ideas in the mind.)
During the second quarter of the 17th century, René Descartes developed a major philosophical and scientific thought which marked a break with all the previous tradition, in particular with the scholastic thought inherited from the Middle Ages. It makes him one of the founders of modern philosophy as well as rational research in science. Although defending throughout his work a dualistic conception of the body-mind relationship, Descartes is also a precursor of idealism by the approach he adopts at the beginning of his Meditations on First Philosophy. Stemming from a critique of the senses, of the imagination and of judgment, quite analogous to that of the skeptics and sophists, Descartes’ idealism starts from his methodical doubt. The philosopher doubts everything, the world and knowledge, in order to better get rid of all prejudice and thus succeed in distinguishing the true from the false. Kant speaks in this sense of “problematic idealism” to qualify the first approach of the Meditations, as opposed to the “dogmatic idealism” which he attributes to George Berkeley.
The methodical doubt of the first of the Meditations leads Descartes to conceive of only one thing that escapes his doubt: doubt itself, the fact that he doubts, therefore that he thinks. His own thought therefore appears to him, once doubt pushed to the extreme limit, as the only indubitable being (Cogito ergo sum, “I think therefore I am”). At this time, Descartes is in the most extreme idealist position: solipsistic idealism, where only the thought of the subject itself exists. But this idealism will only be a provisional idealism in Descartes. Indeed, discovering within his own thought the idea of perfection, he deduces from it that only a perfect being could put in him such an idea, therefore that God exists. And as God incites him to believe in the existence of the external world, divine veracity guarantees him the existence of the external world. This way, Descartes leaves idealism and ends up with a dualistic realism in conformity with the Christian tradition.
Leibniz

(Gottfried Wilhelm von Leibniz around 1695. It was Leibniz who, at the very beginning of the 18th century, initiated the use of the term “idealism” (idealismus))
The German philosopher and mathematician Gottfried Wilhelm Leibniz was the first, in 1702, to conceive the notion of idealism, first for didactic purposes, to oppose the Platonic doctrine of Ideas to the materialism of Epicurus. Although his metaphysics can be described as idealist in at least an ontological and spiritualist sense, Leibniz does not claim idealism himself, for he claims to go beyond the ancient opposition between idealism and materialism by his system based on the concept of monad. The monads are for him the only substantial realities that exist in the universe, and they are spiritual in essence, identifiable with spirits or “proto-spirits”. To justify this conception, Leibniz advances several arguments including the following. On pain of infinite regress, every compound must consist fundamentally of simple elements. Now, since space and time are infinitely divisible, extended matter cannot be simple. In contrast, thoughts, even those with complex content, don’t really have parts, nor do the minds that do. So spirits are the only possible candidates to form the ultimate constituents of reality.
As a good idealist, Leibniz withdrew all substantial existence from the external world understood as a whole, the world of “expand”. But he does not deprive him of all mode of existence. He considers that the mind, although “windowless”, has the legitimate certainty that there is something outside of it without employing the complicated machinery of Descartes’ proof. While of these two self-evident truths, “I think, and there is great variety in my thoughts,” Descartes would have known only the first, Leibniz affirms the value of the second, which “proves that there is something other than ourselves which is the cause of the variety of our appearances,” since one and the same thing cannot be the cause of its own changes. The apparent external world is therefore a “well-founded phenomenon”, because it is established on the necessary existence of the diversity of monads outside of us. If, on the other hand, we consider the order in which these monads coexist and in which they succeed each other, we get space (or “extent”) and time. Far, therefore, from being realities prior to the things of which they would be the receptacles, as the Newtonians believe, space and time are formal structures relating to monads, and therefore to spirits.
Berkeley
(George Berkeley around 1730, first great representative of subjective idealism.)
George Berkeley, philosopher, bishop and Irish theologian, begins very young, as early as 1706 in his Philosophical Notes, to expose the philosophical option for which he remained known, having been seized with the evidence of its principle: “To be is is to be perceived or to perceive” (esse est percipi aut percipere, “esse” being also translated as “to exist”). For him, things which have no faculty of thought – “ideas” – are necessarily perceived, and it is the mind – human or divine – which necessarily perceives them. Divine perception is what maintains the reality of ideas. Matter, understood as a substance distinct and independent of spirit, cannot exist. “Immaterialism”, a term Berkeley himself uses to designate his philosophy, thus became the prototype of idealism for his contemporaries and for his successors, despite his repeated protestations. Following Hegel’s analyzes of the history of idealism, Berkeley becomes the most representative figure of “subjective idealism”, which would have taken the side of the subject as a condition of the very existence of the world.
If Berkeley’s idealism can be considered a subjective idealism, it is in no way an intellectualist or rationalist idealism. For Berkeley, indeed, abstract ideas do not exist; there can only be particular ideas which are perceptions. Berkeley rejects not only intellectualism but also abstractionism of the Aristotelian or Lockean type, that is to say the fact that one can obtain general ideas by erasing the particularities of perceived objects. For Berkeley, a general idea is nothing but the conjunction (“this tree and this and this”) of the perceptions to which it refers; it has no proper and autonomous existence, not even as an object of thought. The notion of abstract idea as accepted in philosophy must therefore be regarded as a “logical monster”, linked to the incorrect use of language. The doctrine that Berkeley aims above all, that of the existence of a thing independent of the mind, is for him a consequence of the faith in abstract ideas.
By its insistence on making perception the only authentic mode of knowledge, and by its rejection of abstract ideas, Berkeley’s idealism must be understood as an empirical idealism, announcing certain radical currents of empiricism such as phenomenalism and neopositivism. Its theocentric character, however, limited its influence.
(Includes texts from Wikipedia translated and adapted by Nicolae Sfetcu)
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