To look at models as &#x27;blurry&#x27; is misleading.  Models are imperfect, but the imperfect model can be more crisp than the underlying reality.<p>Newtonian physics are very useful and less blurry than reality.<p>More relevant, <i>categories</i> are useful and less blurry than reality.  There doesn&#x27;t need to be a sudden major difference between a chicken and a not-chicken.  They only need to have a clear distinction in the model.<p>For comparison, let&#x27;s look at integers.  11 is very similar to 12, and 13 is at least as similar, but 11 and 12 are not-teen and 13 is teen.  There&#x27;s no ambiguity, there is only the definition of the category&#x2F;model &quot;teen&quot;.<p>The distinction is arbitrary, but there&#x27;s still a distinction.  You can easily &quot;draw a line&quot; as the sibling comment says.