QuestionsDo animals matter as much as we do?Between man and animal, a difference of degree and not of kind
Do animals matter as much as we do?A duty of compassion

Between man and animal, a difference of degree and not of kind

Continuity of human and animal minds; humanity to animals

Charles Darwin 1809 · 1882Living things are explained by natural selection, without a prior plan.
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What it claims

Humanity to animals is a virtue and a duty of sympathy: widening sympathy to include them is moral progress.

Why

Animals feel pleasure, pain, love and memory as we do, to a lesser degree: our widening sympathy should therefore extend to them.

In short

"Between man and animal, a difference of degree and not of kind" is an argument by Charles Darwin (Darwin, The Descent of Man, 1871, ch. II-III (III-IV in the 2nd ed., 1874)) on the question "Do animals matter as much as we do?" (answer: a duty of compassion). Its claim: Humanity to animals is a virtue and a duty of sympathy: widening sympathy to include them is moral progress. According to AskPhilosophy's logic check (Z3 solver), the argument is valid only if one grants a premise the author leaves unstated (P4, P5). AskPhilosophy presents 3 objections, 1 of them with a documented reply, and the original text behind each premise, checked word for word.

Darwin, The Descent of Man, 1871, ch. II-III (III-IV in the 2nd ed., 1874)
Valid with an implicit premise

Many think a wall separates humans from animals: reason, language, morality. Darwin argues the opposite. In The Descent of Man, he wants to show that there is no fundamental difference between man and the higher mammals in their mental faculties. Animals feel pleasure and pain, happiness and misery; in them we find, to a lesser degree, love, memory, attention, curiosity. The difference is great, but of degree, not of kind. He adds a moral idea: human sympathy widens little by little, first to family, then to the nation, then to all men, then to all beings that feel. Humanity to animals is one of the latest virtues acquired, and one of the noblest.

P1The lower animals, like man, manifestly feel pleasure and pain, happiness and misery.
P2The difference in mind between man and the higher animals, great as it is, is one of degree and not of kind: love, memory, attention, curiosity and reason are found in them in an incipient or sometimes well-developed state.
P3Moral sympathy extends first to those close to us, then to all men, and finally "beyond the confines of man": humanity to animals is one of the latest moral acquisitions, arising from sympathies becoming more tender and more widely diffused, until they are extended to all sentient beings.
P4 · implicit premiseWhat we owe a being does not depend on a difference of kind that would set it apart from us, but on what it feels: if animals feel as we do, to a lesser degree, our sympathy is owed to them.
P5 · implicit premiseExtending sympathy to beings to whom it is owed is moral progress.
Therefore humanity to animals is a virtue and a duty of sympathy: widening sympathy to include them is moral progress.

click a premise to reject it

If you accept every premise, logic forces you to accept the conclusion. So the debate is about the premises.
Checked by a logic solver: the premises cannot all be true while the conclusion is false.
Without the implicit premise: Without P4, the computer finds a case where animals feel as we do but we owe them nothing: for instance if only beings capable of reason have claims upon us.
The argument is valid with the hidden premise P4. P1 and P2 establish that animals feel as we do, to a lesser degree. P4 links this fact to the duty of sympathy. P3 gives the historical frame: sympathy is already widening, and animals are the next step. Objections target P2 (does language draw a boundary?) and P4 (are duties to animals direct?).
Analogy. Does a river separate two countries, or two banks that gradually meet? Darwin says the "gulf" between man and animal is a gentle slope, not a cliff. Once you see there is no cliff, there is no reason to stay on the bank of indifference.

Objections

3

Each objection is a conversation: click to see who replies and where the debate stands.

René Descartes (language, the boundary between man and animal) 1637 · attacks premise 2

Even the dullest men, not excepting idiots, can join words together to make their thoughts understood, and no other animal, however perfect, does the like. For Descartes, this is the mark of a difference of kind, not of degree.

a defender replies
Charles Darwin defender of the argument · 1871
Darwin admits that the habitual use of articulate language is peculiar to man, but notes that man also uses, like the animals, inarticulate cries, gestures and facial movements to express his meaning, especially the simple and vivid feelings.
Where the debate stands (our summary)Our summary: Descartes and Darwin part ways on language. For Darwin it is the slow fruit of a half-art, half-instinct; for Descartes it proves a thinking soul. Even if language is peculiar to man, it remains to be seen whether it matters for deciding who deserves our sympathy.
Immanuel Kant (indirect duties regarding animals) 1797 · attacks premise 4

For Kant, cruelty to animals is contrary to man's duty to himself, because it blunts compassion and weakens a disposition favourable to morality. They may be killed quickly, without pain, and made to work within their strength. Gratitude for an old horse or dog is an indirect duty: it is owed to oneself, not to the animal.

no reply
We have not yet found a direct reply by Darwin in the texts.
The debate goes on (our summary) Our summary: Darwin quotes Kant on duty, but does not discuss his thesis on animals. Both condemn cruelty and link pity for animals to man's moral character. They differ on the foundation: for Kant, a duty of man to himself; for Darwin, a sympathy widening toward beings that feel.
Where the debate stands (our summary)Our summary: disagreement on the foundation more than on conduct. Kant's defenders say reason alone grounds duties; Darwin's say that the capacity to suffer is enough to make the animal a direct object of sympathy.
A
Alfred Russel Wallace (faculties that selection does not explain) 1870 · attacks premise 2

Wallace, who discovered natural selection at the same time as Darwin, refuses to apply it fully to the human mind. The peoples he calls "savages" have, he says, a brain nearly equal to a philosopher's, with capacities (abstract reasoning, ideas) their life does not need. Selection could only have given them a brain a little superior to an ape's: so a superior intelligence has guided the development of man. The gap is not explained by a mere slope.

no reply
Darwin did reply, but that text is not yet in our corpus.
The debate goes on (our summary) Our summary: Darwin disapproved of Wallace's retreat and maintains, in The Descent of Man, that these faculties formed little by little; his detailed reply is in chapters missing from our corpus. Wallace's word "savages" is that of his time. Even if Wallace were right about the origin of the mind, this does not show that animals do not feel: the objection touches P2, not P1.
Where the debate stands (our summary)Our summary: today's biology follows Darwin on the natural origin of the human mind; whether language and reason make a difference of kind is still debated in philosophy.