when we use that's good and it's good 

Good and Bad, in the sense in which the words are here intended (which is, I believe, their usual sense), are ideas which everybody, or almost everybody, possesses. These ideas are apparently among those which form the simplest concstituents of our more complex ideas, and are therefore incapable of being analysed or built up out of other simpler ideas. When people ask “What do you mean by ‘good’?” the answer must consist, not in a verbal definition such as could be given if one were asked “What do you mean by ‘pentagon’?” but in such a characterization as shall call up the appropriate idea to the mind of the questioner. This characterization may, and probably will, itself contain the idea of good, which would be a fault in a definition, but is harmless when our purpose is merely to stimulate the imagination to the production of the idea which is intended. It is in this way that children are taught the names of colours; they are shown (say) a red book, and told that that is red; and for fear they should think “red” means “book”, they are shown also a red flower, a red ball, and so on, and told that these are all red. Thus the idea of redness is conveyed to their minds, although it is quite impossible to analyse redness or to find constituents which compose it.In the case of “good”, the process is more difficult, both because goodness is not perceived by the senses, like redness, and because there is less agreement as to the things that are good than as to the things that are red. This is perhaps one reason that has led people to think that the notion of good could be analysed into some other notion, such as pleasure or object of desire. A second reason, probably more potent, is the common confusion that makes people think they cannot understand an idea unless they can define it—forgetting that ideas are defined by other ideas, which must be already understood if the definition is to convey any meaning. When people begin to philosophize, they seem to make a point of forgetting everything familiar and ordinary; otherwise their acquaintance with redness or any other colour might show them how an idea can be intelligible where definition, in the sense of analysis, is impossible.

To explain what we mean by Good and Bad, we may say that a thing is good when on its own account it ought to exist, and bad when on its own account it ought not to exist. If it seems to be in our power to cause a thing to exist or not to exist, we ought to try to make it exist if it is good, and not exist if it is bad. When a thing is good, it is fitting that we should feel pleasure in its existence; when it is bad, it is fitting that we should feel pain in its existence. But all such characterizations really presuppose the notions of good and bad, and are therefore useful only as a means of calling up the right ideas, not as logical definitions.

It might be thought that “good” could be defined as the quality of whatever we ought to try to produce. This would merely put ought in place of good as our ultimate undefined notion; but as a matter of fact the good is much wider that what we ought to try to produce. There is no reason to doubt that some of the lost tragedies of Aeschylus were good, but we ought not to try to re-write them, because we should certainly fail. What we ought to do, in fact, is limited by our powers and opportunities, whereas the good is subject to no such limitation. And our knowledge of goods is confined to the things we have experienced or can imagine; but presumably there are many goods of which we human beings have absolutely no knowledge, because they do not come within the very restricted range of our thoughts and feelings. Such goods are still goods, even though human conduct can have no reference to them. Thus the notion of good is wider and more fundamental than any notion concerned with conduct; we use the notion of good in explaining what right conduct is, but we do not use the notion of right conduct in explaining what good is.