What was the grimke sisters argument for equal rights for women based on?

The Grimke sisters were prominent figures for women's rights and very passionate in their Abolition cause. Their arguments were based on the morality of man. Back then, it was morally and socially acceptable for a man to possess a slave and beat and mistreat him. The sisters grew to despise this norm after witnessing firsthand the bad effects it brings to the unpaid workers.