Idealism (Plato): Reality is always changing, senses are limited, so, knowledge (episteme) of reality through the senses is not guaranteed. Then there is opinion, (doxa).
Knowledge is acquired through an exercise of Reason. These are the Forms.
Reason is used to discover unchanging forms through the dialectical method, a process of question and answer designed to elicit a "real definition," i.e. necessary and sufficient conditions for the concept to apply.
Skepticism: An attitude of suspension to the possibility of knowledge or absolute knowledge.
Also known as Pyrrhonism, it takes its name from Pyrrho of Elis (c. 365–275 bc). Pyrrhonists, while not asserting or denying anything, attempted to show that one ought to suspend judgment and avoid making any knowledge claims at all. The Pyrrhonist’s strategy was to show that, for every proposition supported by some evidence, there is an opposite proposition supported by evidence that is equally good.
Faith and reason: A fundamental discussion throughout the Middle Ages is the dichotomy between faith and reason. Faith takes St. Paul's definition: "... faith is the assurance of what we hope for and the certainty of what we do not see." There are three moments: 1- Emphasis on faith over reason in the early patristic theology 2- both faith and reason become complementary in St. Thomas Aquinas, 3- with William of Ockham faith and reason are not related.
Rationalism: (Spinoza, Leibniz, Descartes) In epistemology, rationalism is the view that regards reason as the chief source and test of knowledge. Rationalism is also a methodology or a theory in which the criterion of the truth is not sense-based but instead deductive.
Descartes believed that scientific knowledge can be derived a priori from "innate ideas" through deductive reasoning. Descartes distinguishes two sources of knowledge: intuition and deduction. Intuition is an unmediated mental "seeing," or direct apprehension. Descartes’s intuition of his own thinking guarantees that his belief that he is thinking is true.
Empiricism: Empiricism is the idea that the origin of all knowledge is sense experience. It emphasizes the role of experience and evidence, especially sensory perception, in the formation of ideas. Instead of Cartesian "innate ideas," humans have a posteriori knowledge (i.e. based on experience).
Empiricists believe in inductive reasoning (making generalizations based on individual instances) in order to build a more complex body of knowledge from these direct observations. This is the basis of modern science, and the scientific method, is considered to be methodologically empirical in nature, relying as it does on an inductive methodology for scientific inquiry.
Kantianism: Kant provide a synthesis between Empiricism and Rationalism.
1. Kant separates synthetic and analytic knowledge, and a priori versus a posteriori knowledge.
2. For Kant space and time are "pure intuitions" by which perception can take place, so they are a priori and universal. 3. Our mental abilities only give us knowledge of appearances ("phenomena") and not things-in-themselves ("noumena"). 4. Like Space and Time, Kant defines what he calls Categories which are universal to all conscious entities.
Below some of these categories: