Psychology is considered a science, because it utilizes scientific methodology for devising treatments and measuring outcomes. While some factors of psychology are difficult to measure, the field is rooted in testing hypotheses. Psychological study aims to uncover laws and truths.
Part of psychology is a subset of the field of medicine, and, like medicine, its literature primarily consists of published papers outlining ideas and testing them. Psychology and medicine explicitly overlap in certain fields; antidepressants, for example, are medicines that have been shown to work using standard studies and research. The clinical aspect of psychology has clear scientific roots.
As a whole, the field of psychology is rooted in scientific methodology. The goal of psychology is to determine how the physical brain and the emergent mind work, and this involves tested hypotheses. Testing certain hypotheses is difficult, and moral restrictions limit the means by which hypotheses can be tested. However, the field, along with neurology, has advanced significantly over the years.
Some areas of psychology are criticized for not being based on standard scientific practices. Evolutionary psychology, in particular, posits hypotheses that cannot be tested. The potential promise the field holds, however, makes it a very popular area of study.