Interior Act: The intention or motive behind the act, which must be good (e.g. giving to charity because you want to help others).

Exterior Act: The external act you can see, which must also be good (e.g. giving to charity).

People must perform a good exterior act AND do so with the right intention (interior act) if they wish to glorify God.

 

Exterior Act – The external act a person can see. Internal Act – The motive. Overall is the Act Good or Bad?
Helping an old lady across the road. (GOOD) This person is doing this because they want to impress a friend. (BAD) BAD – He is doing this for selfish reasons.
Helping an old lady across the road. (GOOD) This person is doing this because they genuinely want to help an old lady. (GOOD) GOOD – He is performing the act and glorifying God.

 

Principle of double effect:

Aquinas stated that even if a good act which has an intended good effect  has secondary bad consequences, it is still right to carry out that act. This is known as the principle of double effect. What is important is your intention when performing the act and you cannot be held responsible for any unintended consequences of an act.

For example, a doctor had to perform a hysterectomy (because there was no other choice) to remove a cancerous uterus from a pregnant woman in order to save the woman’s life. This act also led to the death of the foetus, but this would still be acceptable, as the original intention was to save the woman’s life and killing the foetus was an unintended by-product of the action.

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