I just started reading Mortimer Adler's "Aristotle for everybody". In chapter 1, Adler is discussing Aristotle's distinction between living and non-living bodies and plant and animal bodies. He says that Aristotle was fully aware of the "borderline" cases, which are bodies that are hard to classify into one or the other category. One of such would be shellfish, which is hard to classify as an animal if we only see that, like plants, it lacks the power of locomotion.

Despite the borderline cases, Adler says, Aristotle insisted in those distinctions because he reasoned:

If we did not, in the first place, recognize and understand the clear cut distinction between a stone and a mouse, we would never find ourselves puzzled by whether something difficult to classify was a living or a nonliving thing.

This reasoning interests me quite a bit and I would like to read more about it, if possible, in Aristotle's own words. Can you please help me find where in his works I can find this?

  • Does Adler not say what he was referring to?
    – Boba Fit
    Commented Nov 10, 2022 at 14:56
  • @BobaFit he does not give the reference
    – user63395
    Commented Nov 10, 2022 at 14:59
  • 2
    Shellfish are discussed in Historia animalium. Commented Nov 10, 2022 at 15:51
  • @MauroALLEGRANZA thanks
    – user63395
    Commented Nov 10, 2022 at 16:18

1 Answer 1


"A third genus is that of the ostracoderms or 'testaceans'. These are animals that have their hard substance outside and their flesh-like substance within, and their hard substance can be shattered but not crushed; and to this genus belong the snail and the oyster.


"A property common to the above mentioned, and, in fact, to all testaceans, is the smoothness of their shells inside. Some also are capable of motion, like the scallop, and indeed some aver that scallops can actually fly, owing to the circumstance that they often jump right out of the apparatus by means of which they are caught; others are incapable of motion and are attached fast to some external object, as is the case with the pinna."


"As for hardness, softness, toughness and fragility and all the other such affections present in these ensouled parts, these affections might be produced by heat and cold, but not the defining character (logos) in virtue of which the one part is flesh and the other bone; that is the result of the movement derived from the generator, being in actuality what that out of which it comes to be is potentially, just as in things that come to be by craftsmanship."

-Aristotle, A History of Animals

See also his passages on Spontaneous Generation, within his picture of the soul as system.

Aristotle's picture was hylomorphism, form is what unifies some matter into a single object, which is compound of the two. In his 'De Anima' he treats the soul and body as a special case of form and matter. And analyses perception as the reception of form without matter (like, he suggests in his 'Politics' that a constitution is the form of a polis and the citizens its matter).

Aristotle differed with Plato, in seeing forms as not seperable from material or embodiment, except in the minds of a perceived where they are embodied there.

"On the other hand, things are said to be named 'univocally' which have both the name and the definition answering to the name in common. A man and an ox are both 'animal', and these are univocally so named, inasmuch as not only the name, but also the definition, is the same in both cases: for if a man should state in what sense each is an animal, the statement in the one case would be identical with that in the other."

-Aristotle, The Categories

That is they both move, unlike the artwork of an animal.

He gives his picture of the role of humours in On Generation and Corruption. I like the point that aquaducts were a key technology allowing the growth of cities in Aristotle's era, and just as clockwork and steam-engines have subsequently given us cosmological metaphors, we can understand aquaducts as a master-metaphor behind the elements and balancing of humors.

I haven't found a passage directly addressing edge-cases. My sense is Aristotle was comfortable with things being by degree, in terms of their possession of his categories of supervenient soul, except that unique to humans. He saw shellfish as spontaneously generating from mud and sand, according in their proportions to a divine design. So for him living and non-living can be blurred categories, through the action of humors, and forms like bubbles.

You must log in to answer this question.