Animal use has been a hotly contested moral issue for hundreds of years. In the 17th century René Descartes, a French philosopher, argued that animals were no more than automata and could not feel pain. This was rejected by Jeremy Bentham in the 18th Century who extended his utilitarian conception of rights to animals due to their capability to suffer. This was expanded by Peter Singer, in 1975, who wrote in Animal Liberation that arbitrarily treating humans above animals – particularly in marginal cases where animals may be as intelligent as young children or severely cognitively-impaired adults – was not justified.