Interactionism argues that the mind is separate from the material that encompasses it. Living organisms all have a sort of spirit or entity that is above the matter that makes up its body. This spirit or entity is unable to be described in material terms. In humans it is expressed as a mind. It argues how the mind is separate from the body and is not reliant on responses to external stimulation that sends signals to the brain to produce certain types of responses. It argues that the mind is able to act on it's own terms.
Although people respond to external stimuli, we also respond in ways that show an aim for a situation that does not yet exist. If there is purposiveness, it implies that living things approach and work for a purpose which has not yet been achieved, therefore in the mind there is a goal that is being reached for. If this is true, a mind is needed for it to happen. Impulses that trigger our instinctive needs overcome the natural way a living organism responds to external stimulation. For example when salmon release their eggs, they travel upstream in a rivers in order to protect their offspring. There is a purpose to their action that overcomes their natural response, which is to swim with the stream. A creature's instinctive impulse can only be explained by the assumption that the creature is fulfilling the needs for a purpose.
Purposive behavior is a conscious understanding to the human that pursues it. The reason why a person studies for a test is not because of external stimulation, it is because of a purposive goal of passing or doing good on a test. It is impossible to see why the non-existent achievement will stimulate the body unless there is a mind that can foresee the purpose or goal.
Humans are able to appreciate meaning, we see black marks on a white background as something meaningful. It has an impact on our understanding, it is not merely a response to a physical stimuli.