There are also demonstrative or ostensive definitions, which are concepts we can convey by mere shared observations: “that is red” while pointing at a red object, for instance. Potter Stewart famously defined pornography in this manner by saying “I know it when I see it” (Stewart 1964). Knowledge by ostension may reflect epistemic access to a natural kind, although this may feel indistinguishable from an internalized cultural category. There is huge variation among what scientists consider ‘life,’ even among objects on Earth, like viruses and prions, which suggests this kind of definition is not viable for this target.