1. in psychology and other empirical sciences, the concept that events or sequences of events can be predicted with a high, though not perfect, degree of probability and validity on the basis of rational and empirical data. In statistical hypothesis testing associated with empirical research, probabilism is fundamental to the practice of attaching a probability to the truth or falsity of the null hypothesis. See also stochastic.

2. in ethical theory, the notion that when solutions to ethical questions are unclear, one should follow the course with the greatest estimated probability of being ethically correct. It is thought that evidence of a high probability of being correct can be found in agreement among persons of respected moral judgment. —probabilist adj. —probabilistic adj.