Language – Ipse Dixit

Blacks Law Dictionary (10^th ed, 2014, page 956) says:

[Latin “he himself said it”] … Something asserted but not proved <his testimony that she was a liar was nothing more than an ipse dixit> the phrase is commonly used in court decisions analyzing the admissibility of expert testimony.  A court may reject expert-opinion evidence that is connected to existing data only by the expert’s “ipse dixit”.

Wikipedia says as to the origins of Ipse Dixit:

The Latin form of the expression comes from the Roman orator and philosopher Marcus Tullius Cicero (106–43 BC) in his theological studies De Natura Deorum (On the Nature of the Gods) …, an argument from authority made by the disciples of Pythagoras when appealing to the pronouncements of the master rather than to reason or evidence.

Before the early 17th century, scholars applied the ipse dixit term to justify their subject-matter arguments if the arguments previously had been used by the ancient Greek philosopher Aristotle (384–322 BC).

Ipse-dixitism

In the late 18th century, Jeremy Bentham adapted the term ipse dixit into the word ipse-dixitism.  Bentham coined the term to apply to all non-utilitarian political arguments.

Legal usage

In modern legal and administrative decisions, the term ipse dixit has generally been used as a criticism of arguments based solely upon the authority of an individual or organization. For example, in National Tire Dealers & Retreaders Association, Inc. v. Brinegar, 491 F.2d 31, 40 (D.C. Cir. 1974), Circuit Judge Wilkey considered that the Secretary of Transportation’s “statement of the reasons for his conclusion that the requirements are practicable is not so inherently plausible that the court can accept it on the agency’s mere ipse dixit“.^[8]

In 1997, the Supreme Court of the United States recognized the problem of “opinion evidence which is connected to existing data only by the ipse dixit of an expert”.^[9] Likewise, the Supreme Court of Texas has held “a claim will not stand or fall on the mere ipse dixit of a credentialed witness”.^[10] “[W]hen you come across an argument that you recall the majority took issue with,” U.S. Supreme Court Justice Elena Kagan advised readers of her dissent in 2023’s Andy Warhol Foundation for the Visual Arts, Inc. v. Goldsmith, “go back to its response and ask yourself about the ratio of reasoning to ipse dixit.”^[11]

In 1858, Abraham Lincoln said in his speech at Freeport, Illinois, at the second joint debate with Stephen A. Douglas:^[12]

I pass one or two points I have because my time will very soon expire, but I must be allowed to say that Judge Douglas recurs again, as he did upon one or two other occasions, to the enormity of Lincoln,—an insignificant individual like Lincoln,— upon his ipse dixit charging a conspiracy upon a large number of members of Congress, the Supreme Court, and two Presidents, to nationalize slavery. I want to say that, in the first place, I have made no charge of any sort upon my ipse dixit. I have only arrayed the evidence tending to prove it, and presented it to the understanding of others, saying what I think it proves, but giving you the means of judging whether it proves it or not. This is precisely what I have done. I have not placed it upon my ipse dixit at all.

…

The UK Supreme Court said recently in Tui  v Griffiths[2023]UKSC 48 (29 November 2023), Lord Hodge, Deputy President –  with whom Lords Lloyd-Jones, Briggs, Burrows and Stephens agreed, said, when discussing the rule in Browne v Dunn:

[63] Thirdly, there may be a bold assertion of opinion in an expert’s report without any reasoning to support it, what the Lord President (Cooper) in Davie v Magistrates of Edinburgh described as a bare ipse dixit. But reasoning which appears inadequate and is open to criticism for that reason is not the same as a bare ipse dixit.

See the ‘Advocacy’ section for a discussion on Tui  v Griffiths.