I have read here that there is no consensus about the usage of a priori as italic or not. However, increasingly it is used as non-italic.

And here I read the following text:

"In fact the a priori reasoning is so entirely satisfactory to me that if the facts won’t fit in, why so much the worse for the facts is my feeling. [Erasmus Alvey Darwin in a letter to his brother, Charles Darwin (1859)]"

In my own text in a submitted paper, I have the following sentence:

"Additionally, the a priori probabilities in our work are not required to be equal"

The editor called "the a" as a typo. This is not important at all because the work is fairly a technical work and such grammatical issues are not the most important in such works.

However, I really wonder, whether the usage "the a priori..." is correct and if not how one could resolve the issue.

  • 4
    Is it indelicate to suggest that the editor doesn't know the expression a priori and thinks you have two articles back-to-back? – StoneyB on hiatus Dec 6 '15 at 4:47
  • @StoneyB Normally it shouldnt be a problem but editors usually think that they know everything right and a simple explanation about it will be "my opinion" for him. It may be better to provide a reliable text where there is similar usage. I have only that wiki article and editors mostly do not like wiki=) – Seyhmus Güngören Dec 6 '15 at 4:53
  • What's your field? (I ask so we can track down something convincing to your editor, not educated-people-in-general) -- I take it you are looking for "the a priori", not merely unitalicized "a priori". – StoneyB on hiatus Dec 6 '15 at 5:03
  • @StoneyB my field is engineering, basically electrical engineering; signal processing, decision and estimation theory etc. – Seyhmus Güngören Dec 6 '15 at 5:15

I Google-Book "the a priori" signal processing and come up with at least 8 pages of definite hits. Here's one in which "the a priori" is conveniently unitalicized:

enter image description hereTirarenko, Larysa and Barkalov, Alexander, Methods of Signal Processing for Adaptive Antenna Arrays, Springer, 2013, ‘4.2 Nature of a Priori Uncertainty about Properties of Signal and Noise’

There's lots more where that came from, if Google Books gives you anything like what it gives me.

Oh, in case your editor is xenophobic, there are plenty of authors with unexceptionably Anglo names among those hits.


You don't have a usage problem; you have a typesetting problem, which is (like punctuation) a matter of style. Thus you (and your editor) should be guided by your manual of style. I use the Chicago Manual of Style, which recommends roman type for familiar foreign words; italic, for not-so-familiar. As the CMS notes:

In deciding whether or not to italicize, the author's and editor's task is to place the word on the spectrum of usage stretching form foreign-and-unfamiliar at one end to unforeign-and-familiar at the other. And for doing this there are no rules but sensitivity and common sense.

But you have an additional consideration. Typesetting (like punctuation), should encourage readers to parse your text correctly. English does not concatenate articles, so "the a" is likely to jar your readers. As a kindness to them, avail yourself of italics:

In fact the a priori reasoning is so entirely satisfactory to me that if the facts won’t fit in, why so much the worse for the facts is my feeling.

  • Very sound advice, even if I regret its necessity. – StoneyB on hiatus Dec 6 '15 at 5:38

To simply omit "the" or "a" or both, as the editor seems to suggest, would be an error. In some fields, a priori probabilities are also called prior probabilities (or just priors, but that's irrelevant to your question). So you could delete "a" and change "priori" to "prior" (and keep "the" and not switch to italics).

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