Is &quot;I know it when I see it&quot; an &quot;exception that proves the rule&quot;?
<p>US Supreme Court Justice William T. Goldberg is quoted in Wikipedia's <a href="https://en.wikipedia.org/wiki/I_know_it_when_I_see_it" rel="nofollow noreferrer">I know it when I see it</a> as follows:</p>

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  <p>The expression became one of the best-known phrases in the history of the Supreme Court.</p>
  
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    <p>I shall not today attempt further to define the kinds of material I understand to be embraced within that shorthand description, and perhaps I could never succeed in intelligibly doing so. But I know it when I see it, and the motion picture involved in this case is not that.<sup>1</sup></p>
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<p><sup>1</sup>378 U.S. at 197 (Stewart, J., concurring)</p>

<p>and Wikipedia's <a href="https://en.wikipedia.org/wiki/Exception_that_proves_the_rule" rel="nofollow noreferrer">Exception that proves the rule</a> begins</p>

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  <p>"The exception that proves the rule" (sometimes "the exception proves the rule") is a saying whose meaning is contested. Henry Watson Fowler's Modern English Usage identifies five ways in which the phrase has been used, and each use makes some sort of reference to the role that a particular case or event takes in relation to a more general rule.</p>
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<p><strong>Question:</strong> I don't mean to invoke the <em>exceptio probat regulam in casibus non exceptis</em> usage, but instead would like to know if Justice Goldbergs words above have been used as an example of something that while it seems to have worked in this case, is an argument that generally shouldn't be used in rulings.</p>

<p>In other words, a scholarly version of "don't try this at home".</p>

<p>I'm looking for citations and examples of scholarly works or other works of law for examples, and definitely not asking for users' opinions as answers. </p>

<p>Note that Justice Goldberg also qualified that (re-quoted from the same source):</p>

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  <blockquote>
    <p>This simple phrase, embedded in a plurality opinion, carries with it many of the conflicts and inconsistencies that continue to plague American obscenity law. In effect, “I know it when I see it” can still be paraphrased and unpacked as: “I know it when I see it, and someone else will know it when they see it, but what they see and what they know may or may not be what I see and what I know, and that’s okay.”</p>
    
    <p>— William T. Goldberg</p>
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<p>If you are asking if "I know it when I see it" is a popular formally articulated legal standard, it is not.</p>

<p>This said, there are myriad circumstances in which the law entrusts a finder of fact (i.e. a judge or a jury as the case may be) with making a legal determination under a broad and general standard with few formal guidelines.</p>

<p>So, while the phrase is uncommon in precedent (and doesn't apply in obscenity cases either any longer), a broad grant of discretion that boils down to an opinion based decision is common.</p>

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