G. E. Moore's Principia Ethica was published in 1903. In the book Moore defends four theses. The first two are meta-ethical, about the nature of good, whereas the third and fourth express his first-order evaluative views about which acts are right and which things are good.
The first thesis is that goodness is the fundamental ethical notion. The fundamental nature of good for ethics means that it cannot be defined with reference to other ethical notions. Moore thus rejects the dominant intuitionist view that good can be defined in terms of ought, and maintains the contrary thesis that ought can be defined in terms of good – that “ought” means “maximizes good”.
The second thesis is that the term “good” refers to a non-natural property, and so cannot be defined in wholly naturalistic (non-moral) terms. If good could be defined in naturalistic terms, then ethics could be subsumed under the relevant natural science. So if good could be defined in wholly psychological terms, ethics would be subsumed under psychology; and if it could be defined in evolutionary terms, then ethics would be subsumed under biology, and so on. Since good cannot be defined in moral terms either – this follows from his first thesis – Moore concludes that the term “good” must be indefinable, and the property it refers to must be simple.
The third thesis is that there is an irreducible plurality of good things.