The philosophical efforts of the Enlightenment thinkers were based on the relationship between metaphysics and epistemology. Descartes, Leibniz and Berkeley progress in their understandings of being by refining the means by which they are able to make justified claims to knowledge. In a conversation framed around Berkeley's to be is to be perceived, one could phrase the Cartesian rational dualism as to be is to apperceive,' and the Leibnizian rational idealism as to be is to perceive.' These three formulations typify each philosopher's combination of metaphysics and epistemology while demonstrating how each wrote in response to the connected historical discourse in which they participated. The rationalist epistemology employed by Descartes and Leibniz gives way to the empiricism of Berkeley in his attempt to ground abstract epistemological claims in actual experience.
[...] Souls are “images of the universe of creatures, but minds are also images of the divinity itself With respect to qualities of monads, Leibniz equates action with distinctness of perception, and passion (passivity) with confusedness of perception (239b). Thus the active agency of God (in Berkeley's terminology) is a quality of the distinctness of perception proper to His place as the limit on the monadological continuum, and those beings less able than him are endowed with apperception to the degree of their lesser activity. [...]
[...] but by the intellect alone, and that they are not perceived through their being touched . but only through their being understood The process of rational epistemology facilitates sensory perception of body. In the philosophy of both Descartes and Berkeley, the perception of body is thus dependent on a rational mind to perceive sensory ideas. While both philosophers use the interaction of body and idea to explicate their greater projects, the meaning of ‘idea' itself differs in the Cartesian and Berkeleyan epistemologies. [...]
[...] Rationalism, being the method of ascertaining the validity of claims to truth via chains of logic alone, reifies ideas only insofar as they represent a reality that exists more actually in a state external to the idea. In contrast, because one experiences ideas of things without having to reason through representations, it follows that through an empiricist epistemology one would validate only the characteristics of ideas that can be perceived (i.e. experienced). Thus for Berkeley, to perceive an idea as clusters of qualities in the mind is to experience the only possible empirical reality. [...]
[...] Thus, existence of an idea consists in being perceived Berkeley's arguments lead up to the conclusion that to be is to be perceived. Descartes, however, argues that to be is to apperceive. In the second meditation, Descartes writes that exist . for as long as I am thinking To be, in the Cartesian sense, is to perceive oneself being. To observe oneself thinking is to be sure of one's existence: according to Descartes' procedure of doubt, thought is the only attribute of the substance of self that cannot be doubted, because any act of doubting is necessarily an act of thinking. [...]
[...] While this faculty in humans is superior to the perceptive abilities of bare monads (which lack the memory and apperception required to grasp “necessary truths”), the God-monad's perfect distinctness in perception does not require such a faculty of reason as an intermediary between perceiving and understanding. The idealisms of Leibniz and Berkeley, although opposed in epistemological method, share common ground in their ontological privileging of perception. In Berkeleyan metaphysics, for an object appearing external to the mind to be, is to be perceived. [...]
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