Latin expressions in academic and legal English (C1 Vocabulary)

A priori reasons from theory before data, a posteriori from observation, and ipse dixit asserts a claim without proof.

Theory versus evidence

Not "The committee made a priori conclusions from the data." — say "The committee made a posteriori conclusions from the data.". A posteriori starts from evidence; a priori starts from theory.
Try it: Wei: Why did the critic reject her theory? Maya: Its arguments were entirely ______, built from theory with no observation behind them. Choose the words for the blank. (a priori / a posteriori / ab initio) Answer: a priori — Assumptions that come from theory before any observation are a priori.

Word	Pronunciation	Example
a priori	/ˌeɪ praɪˈɔraɪ/	The model depends on a priori assumptions about how rational agents behave.
a posteriori	/ˌeɪ pɑˌstɪriˈɔraɪ/	The report offers an a posteriori explanation of why the intervention actually worked.
ipse dixit	/ˌɪpsi ˈdɪksɪt/	Without a citation, the paragraph reads as ipse dixit rather than argument.

De facto means true in practice, ab initio means from the very beginning, and ipso facto means as an automatic result.

Not "He became the de jure leader, though lacking any official appointment." — say "He became the de facto leader, though lacking any official appointment.". De facto describes status that exists in practice; de jure describes what the law grants.
Try it: Wei: Can the club enforce the agreement? Maya: No, a court treated it as void ______ as it was never validly signed. Choose the words for the blank. (ab initio / de facto / a posteriori) Answer: ab initio — A contract treated as invalid from the very start is void ab initio.

Word	Pronunciation	Example
de facto	/deɪ ˈfæktoʊ/	Although never officially appointed, she became the de facto leader of the project.
ab initio	/ˌæb ɪˈnɪʃioʊ/	The contract was declared void ab initio because consent had been obtained dishonestly.
ipso facto	/ˌɪpsoʊ ˈfæktoʊ/	A serious breach may ipso facto terminate the agreement without further notice.

Mutatis mutandis means with the necessary changes, inter alia among other things, qua in the capacity of, and viz. namely.

Not "The rule applies to remote workers ipso facto, once you adjust for their different hours." — say "The rule applies to remote workers mutatis mutandis, once you adjust for their different hours.". Mutatis mutandis carries a rule over with changes; ipso facto gives an automatic result.
Try it: Wei: Does the same procedure cover online applications? Maya: Yes, ______, with obvious adjustments for the different format. Choose the words for the blank. (mutatis mutandis / ipso facto / inter alia) Answer: mutatis mutandis — Mutatis mutandis means with the necessary changes made, which fits applying a rule to a related case.
Try it: Wei: How did the tribunal treat her rulings? Maya: It assessed her ______ judge and ignored her private academic writing. Choose the words for the blank. (qua / viz. / mutatis mutandis) Answer: qua — Qua means in the capacity of, so it separates her role as a judge from her role as an academic.

Word	Pronunciation	Example
mutatis mutandis	/muːˌtɑːtɪs muːˈtændɪs/	The safety rules for the lab apply, mutatis mutandis, to fieldwork carried out off site.
inter alia	/ˌɪntər ˈeɪliə/	The review criticized the plan for, inter alia, its unrealistic timeline and vague budget.
qua	/kweɪ/	The report assesses her decisions qua regulator, not qua shareholder.
viz.	/vɪz/	Only one department missed the deadline, viz. procurement.

Incorrect	Correct	Why
The committee made a priori conclusions from the data.	The committee made a posteriori conclusions from the data.	A posteriori starts from evidence; a priori starts from theory.
He became the de jure leader, though lacking any official appointment.	He became the de facto leader, though lacking any official appointment.	De facto describes status that exists in practice; de jure describes what the law grants.
The rule applies to remote workers ipso facto, once you adjust for their different hours.	The rule applies to remote workers mutatis mutandis, once you adjust for their different hours.	Mutatis mutandis carries a rule over with changes; ipso facto gives an automatic result.