Psychologists seek knowledge through experience. Psychologists can conduct experiments, observe behaviors, and record their findings just like a professional in any of the "hard sciences" can. Although the inner workings of the mind cannot be directly observed, psychologists can observe what a person says or does. This empirical approach produces substantial facts and evidence to support theories and hypotheses just as in any other science.
Experience is subject to human error because it is contingent upon the psychologist conducting the experiment or observation. Everyone's experience is different, so the results that one psychologist might get from an experiment could be completely different from what another gets from the very same experiment.
[P1] Psychology observes thoughts and behaviors to collect data. [P2] Psychology is a science because psychologists use experience to gain knowledge.
Rejecting the premises
[Rejecting P1] Thoughts and behaviors are poor sources of data because of how unreliably they vary. [Rejecting P2] Psychology can't be considered a science because a psychologist's only source of knowledge (experience) is not fully reliable.