Life doesn't always follow an easy script. In fact, quite often, there are decisions in life when it becomes both necessary and prudent to seek out knowledgeable advice from those who are familiar with the church's teaching on moral issues. One of the most interesting of these moral philosophies is the "principle of double effect", which was articulated by the great Catholic theologian St. Thomas Aquinas, who used it to show that killing in self-defense is justified (Summa Theologiae I-II q64 art. 7).

This principle seeks to add clarity and framework to evaluating whether certain actions are permissible when they produce both desired and undesired consequences. The following are the four necessary conditions which EACH must be satisfied. If the case does not pass all four criteria, it is not permissible.


  1. The moral object (action) must be good or neutral in itself and not intrinsically evil.

    The Catechism of the Catholic Church (1755) says "A morally good act requires the goodness of the object, of the end, and of the circumstances together. An evil end [motive] corrupts the action, even if the object is good in itself (such as praying and fasting "in order to be seen by men"). There are some concrete acts - such as fornication (sex outside of marriage)- that it is always wrong to choose, because choosing them entails a disorder of the will, that is, a moral evil."

  2. The evil result is tolerated, not intended; the good effect is what the agent intends. The agent must have the right intention. (The good must be willed and the evil merely tolerated)

    Catechism of the Catholic Church (1737) says "An effect can be tolerated without being willed by its agent; for instance, a mother's exhaustion from tending her sick child. A bad effect is not imputable if it was not willed either as an end or as a means of an action, e.g., a death a person incurs in aiding someone in danger. For a bad effect to be imputable it must be foreseeable and the agent must have the possibility of avoiding it, as in the case of manslaughter caused by a drunken driver."

  3. The good effect doesn't occur as a result of the evil effect. You can't do an evil to achieve the good; the ends don't justify the means. The good effect must be the direct result of the action taken. The evil effect stems indirectly from the act, while the good effect stems directly from it. (The good effect must not come about as a result of the evil effect, but must come directly from the action itself).

  4. There must be a proportionate reason for doing the act, or the good effect must balance the evil effect. Among these reasons must be the unavailability of other alternatives. (The good effect must be at least equivalent to importance to the evil effect).


Example 1 – A young woman is expecting a child when she is diagnosed with uterine cancer. The cancer is advanced, and without an immediate hysterectomy (removal of the uterus), the expectant mother will die. How is this permissible? A few things to consider:

  1. Removal of a diseased organ is a good.

  2. The intention is to save the mother's life.

  3. Saving the mother's life is not caused by the death of the baby but by the removal of the uterus.

  4. The death of the baby is balanced by the life of the mother.

Example 2 – A person is terminally ill. The family wishes to increase the amount of morphine given in order to lessen the person's pain. The morphine ends up quickening their death. How is this permissible? A few things to consider:

  1. Providing pain medication is a good.

  2. The intention is to lessen pain not bring death faster.

  3. The relief of the pain does not come about as a result of the death, but the reverse.

  4. The death was an unavoidable end, thus the absence of pain balances the quickening of the death.

Example 3 – Carpet bombing in World War II, in which pilots dropped bombs indiscriminately on a city as a scare tactic.

  1. Dropping bombs on innocent people is intrinsically evil.

  2. We can assume they intend good despite the evil (death is intended).

  3. The good effect comes about as a direct result of the evil effect.

  4. The balance is not proportionate.

Example 4 – The parents of Karen Anne Quinlan, who was in a coma, wanted to remove her respirator. Here, the good effect is the placing of a life into nature's or God's hands. The extraordinary means of keeping the person breathing are burdensome and may well be preventing the person's natural time of death. The bad effect is the death itself.

  1. The act of removing extraordinary treatment is neutral.

  2. The parents intend the good effect and tolerate or allow the death.

  3. The death is not caused by the removal of the treatment, but by the body's natural inability to breathe.

  4. Given the burdensomeness of 'forcing' life onto someone whose time to die has likely come, the means are proportionate.

Everyday examples of the principle of double effect would be police officer who risks his life to catch a criminal or a doctor who treats a patient with a contagious disease and risks catching the disease himself. The negative result is not intended, but tolerated.

Chris Stewart — SFA Theologians Guild Member