Distinction , the fundamental philosophical abstraction , involves the recognition of difference. 
In classical philosophy, there were various ways in which things could be distinguished. The actual logical or virtual distinction, such as the difference between concavity and convexity, involves the mental apprehension of two definitions, but it would not be realized outside the mind, as any concave line would be a convex line considered from another perspective. A real distinction involves a level of ontological separation, as when the animals are distinguished from squirrels, or when squirrels are distinguished from squirrels (for not squirrel is a llama, and no llama is a squirrel ).  A real distinction is thus different than a conceptual one, in that it has a distinction, one of the terms can be realized in reality without the other being realized.
Later developments include Duns Scotus’ formal distinction , which developed in part from the recognition in the context of an intermediary between logical and real distinctions. 
Some relevant distinctions to the history of Western philosophy include:
- Necessity and Contingency
- Inductive and Deductive
Distinctions in Contemporary Thought
While there is anticipation of this distinction in Kant in the British Empiricists (and even further in Scholastic Thought), it was Kant who introduced the terminology. The distinction concerns the relationship of a predicate, as in “All bodies are extended.” Synthetic claims bring together two concepts together, as in “All events are caused.” The distinction was recently called into question by WVO Quine , in his paper ” Two Dogmas of Empiricism .”
A priori and a posteriori
The origins of the distinction are less clear, and it concerns the origins of knowledge . A posteriori knowledge arises from, or is caused by, experience. A priori knowledge can be obtained from experience, but its certainty is not derivable from the experience itself. Saul Kripke was the first major thinker proposing that there is analytic a posteriori knowledge claims.
Notable Distinctions in Historical Authors
Aristotle makes the distinction between actuality and potentiality .  Actuality is a realization of the way a thing could be, while potency refers to the way a thing could be. There are two levels to each: Itself matter can be anything, and something Actually Becomes by causes , making it something qui Then Has the Ability to be in a definite way, and That Ability can be Realized Then. The matter of an ax can be an ax, then is made into an ax. The ax is able to cut, and reaches a new form of actuality in actually cutting.
The major distinction Aquinas makes is that of essence and existence . It is a distinction already in Avicenna , but Aquinas maps the distinction to the actuality / potentiality distinction of Aristotle, such that the essence of a thing is in potency to the existence of a thing, which is that thing’s actuality. 
In Kant , the distinction between appearance and thing-in-itself is foundational to its entire philosophical project.  The distinction separates the way a thing appears to the one hand, and the way a thing really is.
- Jump up^ Sokolowski, Robert (1998-01-01). “The Method of Philosophy: Making Distinctions” . Review of Metaphysics . 51 (3): 515-532.
- Jump up^ Copleston, Frederick (2003-06-12). History of Philosophy Volume 2: Medieval Philosophy . A & C Black. ISBN 9780826468963 .
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- Jump up^ Colin, Marshall ,. “Kant’s One Self and the Appearance / Thing-in-itself Distinction” . Kant-Studien . 104 . ISSN 0022-8877 .