J. L. Mackie in the context of "Alvin Plantinga's free will defense"

In philosophy, metaphysical necessity, sometimes called broad logical necessity, is one of many different kinds of necessity, which sits between logical necessity and nomological (or physical) necessity, in the sense that logical necessity entails metaphysical necessity, but not vice versa, and metaphysical necessity entails physical necessity, but not vice versa. A proposition is said to be necessary if it could not have failed to be the case. Nomological necessity is necessity according to the laws of physics and logical necessity is necessity according to the laws of logic, while metaphysical necessities are necessary in the sense that the world could not possibly have been otherwise. What facts are metaphysically necessary, and on what basis we might view certain facts as metaphysically but not logically necessary are subjects of substantial discussion in contemporary philosophy. In Islamic philosophy, it is called "ayan-ı sabite" (The immutable knowledge of beings), assumed to exist in God's knowledge before they exist was; thus defended as immutable in God's knowledge, just as in his body.

The concept of a metaphysically necessary being plays an important role in certain arguments for the existence of God, especially the ontological argument, but metaphysical necessity is also one of the central concepts in late 20th century analytic philosophy. Metaphysical necessity has proved a controversial concept, and criticized by David Hume, Immanuel Kant, J. L. Mackie, and Richard Swinburne, among others.

Moral skepticism (or moral scepticism in British English) is a class of meta-ethical theories all members of which entail that no one has any moral knowledge. Many moral skeptics also make the modal claim that moral knowledge is impossible. Moral skepticism is particularly opposed to moral realism, the view that there are knowable and objective moral truths.

Some defenders of moral skepticism include Pyrrho, Aenesidemus, Sextus Empiricus, David Hume, J. L. Mackie (1977), Friedrich Nietzsche, Richard Joyce (2001), Joshua Greene, Richard Garner, Walter Sinnott-Armstrong (2006b), and James Flynn. Strictly speaking, Gilbert Harman (1975) argues in favor of a kind of moral relativism, not moral skepticism. However, he has influenced some contemporary moral skeptics.

Moral skepticism (or moral scepticism in British English) is a class of meta-ethical theories all members of which entail that no one has any moral knowledge. Many moral skeptics also make the modal claim that moral knowledge is impossible. Moral skepticism is particularly opposed to moral realism, the view that there are knowable and objective moral truths.

Some defenders of moral skepticism include Pyrrho, Aenesidemus, Sextus Empiricus, David Hume, J. L. Mackie (1977), Friedrich Nietzsche, Richard Joyce (2001), Joshua Greene, Richard Garner, Walter Sinnott-Armstrong (2006), and James Flynn. Strictly speaking, Gilbert Harman (1975) argues in favor of a kind of moral relativism, not moral skepticism. However, he has influenced some contemporary moral skeptics.