Cause X is a cause area currently neglected by the effective altruism community, typically due to some form of moral blindness or fundamental oversight, yet more important than all the causes currently prioritized by it.

The idea was introduced by William MacAskill (MacAskill 2016), possibly by analogy with Derek Parfit's "Theory X", a currently unknown hypothetical theory which would solve a number of important open problems in population ethics (Parfit 1984). ("Cause X" is also sometimes used loosely to refer to any promising and neglected cause (Savoie 2019; Gómez Emilsson 2019), but this is not how the expression is generally understood (Rice 2019).)

Does Cause X exist?

The existence of Cause X should not be taken for granted; whether there is a Cause X is an open question. In contrast to Parfit's Theory X, however, there are no impossibility theorems that could disprove the existence of Cause X (Ng 1989). Furthermore, if there is a Cause X, there is no determinate way of ascertaining that it has been found; this is another disanalogy with Theory X, whose conditions are defined with sufficient precision that it is possible to establish when a theory satisfies them....

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