Because of his Transcendentalist philosophies, Henry David Thoreau was anti-slavery, believing that man was intended to find his own destiny and his own spiritual meaning separated from the pressures of society. This meant that Thoreau found slavery inherently immoral, less from its sociological perspectives or general cruelty than for its idea that any man could be owned by another, thus limiting the spiritual growth and personal achievement of the owned man. In Walden, Thoreau writes:

...worst of all [slavery] when you are the slave-driver of yourself! ... Look at the teamster on the highway... his highest duty to fodder and water his horses! ... See how he cowers and sneaks, how vaguely all the day he fears, not being immortal nor divine, but the slave and prisoner of his own opinion of himself...
(Thoreau, Walden, Google Books)

Here, Thoreau equates African slavery to the moral slavery of commerce, "masters" (including simple employment), and of fearing man and public opinion rather than fearing the lack of personal growth he saw in many otherwise-normal men. In his view, the slavery of all men under public opinion was as bad as actual slavery, since it is unconsciously accepted as part of the normal course of things. African slavery was overtly cruel and evil, and could eventually be recognized as such, while the slavery of commerce is ignored and even praised. This view is in line with his general philosophies of individual responsibility, freedom, and divine purpose.