Henry was a writer and campaigner for social reform in the fields of prisons, schools, economic institutions, and the treatment of non-human animals. He was a noted ethical vegetarian/vegan, anti-vivisectionist, socialist, and pacifist, and was well known as a literary critic, biographer, classical scholar and naturalist.  Salt is considered, by some, to be the "father of animal rights," having been an early writer to argue explicitly in favour of the topic, in his "Animals' Rights: Considered in Relation to Social Progress" (1892).

He wrote: "[The] notion of the life of an animal having 'no moral purpose,' belongs to a class of ideas which cannot possibly be accepted by the advanced humanitarian thought of the present day—it is a purely arbitrary assumption, at variance with our best instincts, at variance with our best science, and absolutely fatal (if the subject be clearly thought out) to any full realization of animals' rights."

Henry also wrote extensively on his rationalism and naturalism, saying: "Religion has never befriended the cause of humaneness. Its monstrous doctrine of eternal punishment and the torture of the damned underlies much of the barbarity with which man has treated man; and the deep division imagined by the Church between the human being, with his immortal soul, and the soulless “beasts”, has been responsible for an incalculable sum of cruelty."

Henry on Wikipedia
henrysalt.co.uk
Thanks to @maddyogoodall for the suggestion.