‘A Priori’ and ‘A Posteriori’ Knowledge: What Are They?

According to Immanuel Kant, the great German philosopher of the 18th century, there are two types of knowledge: a priori and a posteriori. Understanding them is the preliminary key to unlocking Kantian philosophy, particularly his works on epistemology and metaphysics.

The etymology of a priori and a posteriori is derived from Latin. The prefix a means ‘by’ or ‘from’, whereas ‘priori’ and ‘posteriori’ are antonyms. Priori means ‘prior’ or ‘before’ whereas posteriori means ‘after’ or ‘later’. Thus, ā priōrī is Latin for ‘from before’ or ‘what comes prior to something else’ and ā posteriōrī is Latin for ‘from later’ or ‘what comes after something else’. Today, a priori and a posteriori are part of the canonical jargon in the field of epistemology. Immanuel Kant was the first to coin and popularize these terms systematically and their employment in modern philosophy goes back to their Kantian understanding. However, he was not the first to use them.

Although used differently, the terms date back to Aristotle, who first employed them in two texts from Organon known as Analytica Priora (Prior Analytics) and Analytica Posteriora (Posterior Analytics) respectively. Aristotle considered them as two types of reasoning, deductive and inductive. In Prior Analytics, Aristotle laid the groundwork of syllogistic logic, which is a logical structure that assumes the certainty of conclusions based on the certainty of premises. For instance, if we consider the premises “Aristotle was a philosopher” and “philosophers are human”, then our conclusion would certainly be “Aristotle is a human”. Posterior Analytics, on the other hand, is a major work on scientific knowledge that explores the process of reaching conclusions based on observation. For example, if we see the sunrise every day, we assume that the sun will rise tomorrow. The former is a structure of rational necessity, whereas the latter is a probability derived from observation.

Kantian Epistemology

Immanuel Kant developed the concept of a priori and a posteriori knowledge in his magnum opus, The Critique of Pure Reason. According to him, a priori is a type of knowledge that is completely independent of empirical observation, but a necessary postulate of ‘pure reason’. Inversely, a posteriori knowledge is entirely dependent on sensory experience. For example, “1+1=2” is an a priori truth, for it is not contingent on empirical observation but a postulate of reason. Of course, we can empirically observe 1+1=2, but what makes it a priori knowledge is that we don’t need to justify it through experience because it is already warranted by reason alone. However, the fact that trees are typically green is a posteriori knowledge, for its justification depends on sensory observation and is not inherent within the structures of our rationality.

Kant employs the Latin meaning of a priori and a posteriori to refer to knowledge that can be reached ‘prior’ to experience and knowledge that can only be acquired ‘after’ empirical observation. But can we really gain knowledge ‘prior’ to experience? Kant himself claims that “ no knowledge within us is antecedent to experience, but all knowledge begins with it”, for otherwise our cognitive faculties won’t be stimulated into activity (Kant, 1781). So, in what way is a priori knowledge precedent to experience? 

Kant argues that a priori knowledge is the inherent structure of our cognitive faculties that shape our experience. He distinguishes between intuition and understanding as modes of a priori knowledge. According to him, time and space are intuitions that are inherent within our cognition rather than truths inherent in our experience. Likewise, he claims that our ability to perceive quantity, indeed forms, qualities, and causation is inherent in the categories of our understanding. These a priori principles come ‘prior’ to experience in the sense that they are not found in our sense impressions, but are that which gives them a coherent structure. We don’t say that a priori concepts are ‘independent’ from experience because they are unrelated to it. On the contrary, as we’ve seen, they are very intimately connected. They are independent from experience because their source of knowledge does not lie in experience, even if it shapes it.