Epistemology, the branch of philosophy that studies knowledge and belief, distinguishes two types of arguments: a posteriori and a priori. A posteriori arguments are those that rely on empirical evidence or sensory experience, while a priori arguments are those that rely on reasoning and deduction independent of sensory experience. In this essay, we will explore the characteristics of both types of arguments.
A posteriori arguments are grounded in empirical evidence, which means that they are based on what we can observe through our senses. This type of argument is also known as inductive reasoning, which means that we move from specific instances to general conclusions. For example, if we observe that all swans we have seen are white, we may conclude that all swans are white. However, the reliability of a posteriori arguments is limited by the possibility of errors in observation or interpretation. As David Hume famously argued, we cannot logically derive general statements from singular observations because we cannot guarantee that future observations will confirm our previous experiences (Hume, 1739/1975). An example of an a posteriori argument is the Design Argument for God's existence. For example
P1 (premise 1) Order and purpose only exist where an intelligent agent has been at work
P2 (premise 2) The world shows signs of order and purpose
C1 (conclusion 1) Therefore, the world must have been designed by an intelligent agent.
P3 (premise 3) God is the only intelligent agent capable of designing worlds
C2 (conclusion 2) Therefore, God exists
A priori arguments, on the other hand, are not grounded in empirical evidence. They are based on reasoning and deduction from self-evident propositions. A priori arguments are often used in mathematics and logic. For example, the proposition that 2+2=4 is a priori since it can be logically derived from the definitions of numbers and addition. A priori arguments are also used in philosophical discussions about metaphysics, ethics, and epistemology. For example, the argument that it is impossible for a perfect being to do evil is a priori, since it is based on the concept of perfection and the nature of evil. A priori arguments are not infallible, however. The validity of an a priori argument depends on the clarity and consistency of the propositions used. The Ontological Argument is a good example of an a priori argument. Descartes’ or the Cartesian version ("Cartesian" means "belonging to Descartes") is as follows:
P1 (premise 1) Order and purpose only exist where an intelligent agent has been at work
P2 (premise 2) The world shows signs of order and purpose
C1 (conclusion 1) Therefore, the world must have been designed by an intelligent agent.
P3 (premise 3) God is the only intelligent agent capable of designing worlds
C2 (conclusion 2) Therefore, God exists
Kant refers to the knowledge gained from this sort of argument as analytical knowledge - it is an improved understanding of what the premises in the argument mean, rather than knowledge based on experience.
For example:
P1: Bachelors are unmarried men
P2: Finlay is a bachelor
C: Therefore Finlay is an unmarried man
A priori arguments focus on the structure of the argument - its validity. If an argument is a priori then it is valid if the conclusions do flow logically from the meaning of the premises.
In summary, the main difference between a posteriori and a priori arguments is the source of their premises. A posteriori arguments rely on empirical evidence, while a priori arguments rely on reasoning and deduction from self-evident propositions. Both types of arguments have their limitations, but they are valuable tools in different domains of knowledge.
References:
Hume, D. (1975). Enquiries concerning human understanding and concerning the principles of morals. Oxford University Press.
Kant, I. (1781). Critique of pure reason. Cambridge University Press.