Most of us would want to say that it is true that Socrates taught Plato. According to realists about past facts, this is made true by the fact that there is, located in the past, i.e., earlier than now, at least one real event that is the teaching of Plato by Socrates. Presentists, however, in denying that past events and facts exist cannot appeal to such facts to make their past-tensed Statements true. So what is a presentist to do?

There are at least three conditions that would ideally be met in a satisfactory solution to this problem:

• a) It must preserve our views about which Statements are true and which false

• b) It must be transparent what the truthmakers are for those Statements

• c) It must accommodate the truth-value links between various times.

Suppose Figure 1 depicts Stimuli from an experiment on shape discrimination, where the subjects are asked to point out the best circle.

Now suppose that Figure 2 shows Stimuli from a color-discrimination experiment where the subjects’ task is to pick the purest green — green that is neither yellowish nor bluish — in other words, is unique green.

In both these tasks there are individual differences between different subjects. However, notice that in the shape-discrimination case there is exactly one correct response: the best circle is the fourth from the left. In the color case it is not obvious, to put it mildly, that there is exactly one correct response. One color-normal subject may find that the purest green is the third from the left, whereas another may choose the fifth from the left, and still another may pick the fourth. Who is right, and who is wrong? More importantly, why is there this difference between shape perception and color perception?

In section 53 of Word and Object, Quine argues that the set-theoretical explications of the concept of the ordered pair of f ered by mathematicians such as Wiener and Kuratowski give us a model for the clarification of philosophically troublesome ideas. According to Quine, ordered pairs might seem indispensible in science. But at the same time they have appeared unclear to many philosophers, who have argued that an extensional treatment of the logic of relations can be satisfactory only to the extent that we can give a transparent and substantial explanation of what an ordered pair really is. Quine cites Peirce as someone who tries to meet this sort of demand. The explanation Peirce offers is mentalistic, but it is not the mentalism that Quine regards as the most fundamental problem with Peirce's account.

It is part of our notion of moral properties (certain forms of relativism to the contrary) that they are in some sense independent of our moral beliefs. A murderer cannot make his action moral simply by believing that it is so. Slavery was immoral even if a large number of people once believed that it was permissible, and it would remain so in the future even if every person came to believe that it was morally acceptable. But views that take moral properties to be objective and thoroughly mind-independent constituents of reality face familiar metaphysical and epistemological obstacles.

In its simplest form, the principle of fairness tells us the following. If a number of people are producing a public good that we benefit from, it is not morally acceptable to free ride on their backs, enjoying the benefits without paying the costs. We owe them our fair share of the costs of the production of that good. The principle of fairness, defended by Rawls in A Theory of Justice and widely discussed subsequently, is sometimes invoked in various areas of applied ethics, such as environmental ethics, social ethics, etc.

As it stands, the principle of fairness offers an account of a particular type of moral obligation that obtains if certain conditions are satisfied. For example, the good produced must be a particular type of good, the scheme in which it is produced must itself be fair, and so on. This account inspired a wave of discussions, which set the tone for the debate on the principle of fairness, about two decades ago.