'Knowing what factors to look at is useless unless one knows what to look for'

Wise words from Judge Diane Sykes, writing for a Seventh Circuit en banc majority in United States v. Mayfield (7th Cir. Nov. 13, 2014):

Multifactor tests are common in our law but they can be cryptic when unattached to a substantive legal standard, as this one is. Knowing what factors to look at is useless unless one knows what to look for. Without a legal definition of predisposition, jurors are left to weigh the listed factors in the abstract, or perhaps to weigh them against an intuitive understanding of the term. Some concepts in our law are appropriately left to the common sense and collective wisdom of the jury. See United States v. Hatfield, 591 F.3d 945, 949-50 (7th Cir.2010) (explaining that the term "reasonable doubt" is best left undefined); United States v. Glass, 846 F.2d 386, 387 (7th Cir.1988) ("Jurors know what is 'reasonable' and are quite familiar with the meaning of 'doubt.'"). The concept of predisposition is not so well understood that it belongs in this category. Our multifactor test for predisposition would be more useful if we defined the term.

The quote is about the "predisposition" element of the entrapment defense, but it makes sense in lots of other cases, too. Consider, for instance, Justice Sandra Day O'Connor's statement in Rosenberger v. Rector (1995) that, "[w]hen two bedrock principles [government neutrality and the prohibition on state funding of religious activities] so conflict," "[r]eliance on categorical platitudes is unavailing" and "[r]esolution instead depends on the hard task of judging—sifting through the details and determining whether the challenged program offends the Establishment Clause." When one is sifting through those details, what exactly should one be looking for? Likewise, when one is applying the copyright fair use factors, "[i]n determining whether the use made of a work in any particular case is a fair use," how does one know how any of the factors bears on the fairness, unless one has an understanding of what "fair" actually means here?