Also called ‘extreme’ or direct utilitarianism.
Original, and ‘official’ form of utilitarianism which says that our duty on any occasion is to act in the way which will produce actual overall consequences better than (or at least as good as) those that any other act open to us would produce.
Difficulties in predicting consequences, including difficulties in principle where self-prediction is concerned, mean that as a practical prescription utilitarianism can only tell us to aim for the best probable outcome, and act utilitarianism has often been superseded by rule utilitarianism.
Source:
J J C Smart, ‘Extreme and Restricted Utilitarianism’, Theories of Ethics, P Foot, ed. (1967); defends the former