What does a priori mean?

In your time reading biology papers, you’ve likely encountered some Latin terms like in vitro, in vivo, de novo, in situ, et cetera. Because the Latin language had such a large influence on the early natural sciences, these terms are still in common use and have mostly retained their original definitions. However, you may have occasionally seen one more phrase whose meaning is a bit more complicated : a priori. This concept is borrowed from philosophy (more specifically epistemology, or theories of knowledge), where it has a long and controversial history dating to at least the 18th century. So what does a priori mean?

Two types of knowledge: A priori vs. a posteriori

In simple terms it can be summarized as follows: There are thought to be two distinct types of knowledge, the first which can be acquired through our deductive reasoning alone (a priori – from the former), and the second which is acquired through our empirical faculties (i.e. sensory data and evidence) in combination with inductive reasoning (a posteriori – from the latter). This second type of knowledge is familiar to all scientists, and forms the foundation of the scientific method; it is used on a daily basis when we perform experiments and then use the results to draw general conclusions.

Let’s look at a hypothetical example: Following a stressful behavioral stimulus, a specific region of the mouse brain is activated (assume that all appropriate control experiments were performed as well). Employing inductive reasoning, it is concluded that all mice of similar genetic background and in similar conditions would display this same pattern of neural activation. This is an example of a posteriori knowledge, because it requires empirical evidence to form the conclusion. So what would be an example of a priori knowledge or reasoning? One classic example is the statement “All bachelors are unmarried.” To confirm the truth of this proposition, there is no need to conduct a survey of all bachelors, since it is true by definition; this is also known colloquially as “armchair” reasoning.

a priori vs a posteriori
Kant’s explanation of the difference between a priori and a posteriori knowledge. Image source: Market Business News.

The natural sciences, like biology and chemistry, require empirical evidence to make truth claims; so what can scientists possibly mean when they use “a priori” in a journal article? This is where the situation becomes more nuanced, and it becomes possible to easily misuse the phrase. In most contexts, I believe the author means something along these lines: “Given our current knowledge and understanding of X, we can make the following argument without requiring any new empirical data.”

What does a priori mean in a scientific context?

To illustrate the point, here is a quotation from a recent paper in Current Biology1, in which bicoid mRNA levels are compared between different D. melanogaster embryos: “A priori, it is possible that the number of source mRNA molecules fluctuates significantly from embryo to embryo.” In other words, given our current scientific knowledge of how gene expression is regulated, it is possible that there are differences in bicoid mRNA levels among different embryos. The authors would then go on to test this possibility by experiment, thus adding to our a posteriori knowledge. Hopefully the above explanation clarifies what a priori means.

If you enjoyed this breakdown of the definition of a priori, please check out my post on how to use p-values in biology.


  1. Petkova, MD, Little SC, Liu F, Gregor T. Maternal origins of developmental reproducibility. Current Biology. 24, 1283-88 (2014). Available at: http://www.sciencedirect.com/science/article/pii/S0960982214004667

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