I have always believed that for something to be science, it must use the scientific method.
The thing about psychology is that it is considered a medical science. Psychologists diagnose and treat patients.
However, they DO NOT use the scientific method to diagnose and treat patients, thus it is not a science.
There are no definitive tests for any mental illnesses. You can't see crazy on an x-ray or find it in a blood test. A person who suffers from mental illness can see 10 different psychologists and get anywhere from 5-15 different diagnosis. If there were a test for mental illness that could be repeated by others in the same field with the same results, THEN and only then would that be science.
In my humble opinion psychology will fall the way of alchemy. Sure alchemy had it's place in the past. It was part science, part chemistry and part hocus-pocus. The same thing could be said about psychology. Advances in neuroscience will replace psychology all together. When you can look at the brain and pinpoint the exact cause of the problem, just like you can look at an x-ray of your leg to see if the bone is broken, then there will finally be some relief for people who suffer from mental illness. THEN it will be science.