A priori, A posteriori, A fortiori: What do they all mean? Philosophy Basics

A priori and a posteriori are common words within philosophy that are introduced to students on the A-Level specification. However, it wasn’t until I started my masters degree that I came across the term a fortiori. But what is the difference between them? And why are a priori and a posteriori paired together without a fortiori?

What does a priori mean?

A priori refers to something which is known without having to appeal to experience. If we were to lock ourselves into a cave and gained knowledge via purely thinking, this knowledge would be a priori. The justification for believing in such knowledge would be independent of experience. This can include:

A) Propositions
We can know 1+1=2 purely via thinking.

B) Arguments
Tautologies are the most common example. These are arguments which arise from definition. We can know that a bachelor is an unmarried man just by looking at a definition. Such arguments can include ontological arguments for the existence of God: if we define God as the perfect being and it is better to exist than not exist, then God must exist.

What does a posteriori mean? How does it relate to a priori?

A posteriori is the opposite to a priori. It refers to arguments or propositions where the justification involves appealing to experience.

A) Propositions
Ravens are black. You would not be able to know this if you hadn’t encountered some form of the concept of raven through interactions with the world.

B) Arguments
Design arguments are the most common (though not always – they can be a mixture of a priori and a posteriori premises.) Essentially, they look to features of the world and argue there are elements signify design. From this, they argue that there must be a designer, who is God. You would not be able to make such an argument without experiencing the world first and seeing the features of design. Thus, it is an a posteriori argument.

Is there any common ground between a fortiori and the previous definitions?

A fortiori refers to justification. However, it does not refer to experience. Instead, it signifies where we have stronger grounds for one argument or conclusion over another. It is similar to, ‘to an even greater extent.’ For instance, if you have a person who is 15 and deemed too young to drink alcohol, then, a fortiori, a person who is 13 is also deemed too young to drink alcohol. With the younger age, there is a greater extent to which that person is too young to drink.

A fortiori can also be used when we reject one conclusion in favor of another because we deem to have stronger justification. For instance, we may accept that God does not exist because, in our eyes, the argument from the problem of evil is a fortiori to any argument which provides alternative explanations as to why there is evil in the world.

Published by Philosopher Ad Absurdum

Student studying MA Philosophy of Mind and Cognitive Science at the University of Birmingham; First Class BA Philosophy and History from the University of Southampton.

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