Double effect principle

The principle of double effect is a “principle of practical reasoning used to determine the legality or illegality of an action that produces or can produce two effects, of which one is good and the other is bad.” 1 “[T] he main idea behind the principle of double effect is that a person is not equally responsible for all the bad effects that follow from their action but there is a fundamental difference between those who try and those who only expected Or must anticipate. ” 2 also can be termed as non – liability principle of indirect evil produced by direct voluntary.


It arises from Thomas Aquinas, and then acquires a range more open, accessible to scholars of ethics in general. Indeed, “from the second half of the twentieth century, with the rehabilitation of classical practical philosophy in the Anglo-Saxon analytic field, the principle is again vigorously studied.” 3

General information 

Although this principle has earlier precedents (for example, in the sum of theology of Thomas Aquinas , II-II, q. 64, a. 7), it was developed more systematically by a French Jesuit, Jean-Pierre Gury (1801 -1886), author of a famous compendium of moral theology.

Gury distinguished between:

  • Principle of accountability of indirect voluntary “in causa”;
  • Principle of non-imputability of the direct volunteer “in causa” with two effects, one good and one bad: it is a voluntary act, one of whose effects is not wanted directly.

Imputability implies three aspects (according to the three dimensions of human action):

  • Knowledge : the agent provides what the effect of their action (albeit confusing or imprecise).
  • Will : the agent is aware that you can not put the act or, once set, can “remove” (stop holding the act).
  • Company : the agent perceives the context of duties in which his act is placed.

The conditions of the principle of double effect 

Scholars have gone deeper into what is now known as conditions that would allow the double-effect principle to be applied properly. According to some enumerations, these would be four or more conditions. Such conditions would be:

  • The object of the act that is pursued must be good.
  • The intention of the one who acts must be good and excludes (he does not want, but tolerates) the bad effect that will follow from the action.
  • The action must be in itself good or indifferent, for it would not be right to undertake an evil act (for example, stealing) to achieve a good purpose (curing ordinary flu).
  • There must be a proportionately serious reason to accept the act. That is, the expected benefit must be serious enough to justify the initiation of an action that will bring some negative consequence (and less negative than the good outcome that is expected to achieve).

To the above conditions a prerequisite can be added: that there are no better actions, that is, that alternatives are not possible that cause less damage than those that will be provoked with the act that will reach a good result and collateral damages.