Theories of the mind are as old as history itself, but its only in the 19th century that doctors and scientists started to study it more carefully. Freud is the first to put the concept of unconscious into a unified framework.
Freud's concept is based on three states of the mind: conscious, pre-concious and unconscious. The conscious part of the mind is what readily available to our thoughts as we percieve them, The preconscious is the part of the unconscious the closest to the conscious. It is readily accessible if we think about a word. For example, the date of Bone Fire Night isn't necessarily in our mind all the time, but we can readily access it when neeeded.
A second part of the unconscious is more difficult to acess and can take up years to bring back to memory. That aspect is hidden to us most of the time and we have to make great efforts to access it.
A third part of the unsconscious is never accessible. In English, we call it the "Id", it is the part that is linked with instincts and has no moral sense. It is the only part that is fully unconscious in the sense that we can't access it directly.
The Id is part of another classification used by Freud including the "ego" and the "super-ego". This classification he realised later in his life is different from the conscious/unconscious approach in the sense that the ego and the super-ego have both a conscious and an unconscious part. They are also more useful for the therapist as, for Freud, it is the dysfonctionnement of these that leads to neurosis.
The ego mediates between reality and mental processes. It is the part that makes us function as human. The super-ego is a remnant of parental injunction, the "conscience, shame and guilt" associated with their recommendation of what one should or should not be.
According to Freud, the therapist role is to discover resistance to access certain memories in order to find out what causes trouble in the patient. Theses resistance are caused by the inability of the ego to find a compromise between the Id/Super-ego and reality. This inability results in repressed memories that are harmful.
In summary, Freud's approach to the mind is one of antagonism: the ego is constantly fighting pulsions form the id and injuction from the super-ego and trying to reconcile them with reality. The role of the therapist is act as an impartial miror (although he sometimes got involved) so that the patient can reveal is own contradiction and repressed memories.
On the other side of the scale, Erickson consider the unconscious - both of the patient and the therapist - as the best ally to the cure of the patient. For him, hypnosis was a way to let the unconscious take over and actually do a better job than the conscious would. He regularly did therapy in transe and used his unconscious' insight to make diagnosis.
In the same way, he considered therapy as a way to put in action the unconscious of the patient as an ally. As the unconscious "develops insight in advance of conscious", it was best to let it work by itself. Erickson would use the symbols created by the unconscious to understand. In some ways like Freud, he considered that some processes were "unseeable and unknowable", but the unconscious gave ideas in the form of metaphors. The role of the therapist is to exrtact meaning from these metaphors, not necessarily explicitely, and to utilize them to help the patient improve.
Erickson's approahc was extremely empirical, as he considered that each patient was unique and necessitated a specific approahc. He refused to develop a theory but relied on principles that, like the metaphors of the unconscious, give us an insight into was he considered the mind to be: a split between a conscious and an unconscious self, with all the tools necessary to better itself. The role of the therapist in that framework was to bring to the attention of the patient what he was able to do, and to use his strengths in order to resolve his problems.
The two previous theories of the mind are based on experience and personal thoughts of the creators. They are based on theories of the mind that are entirely separated from the physical object that is the source of the thoughts. Another multiform approach consists in associating behaviour (mental or physical) with locations in the brain. With phrenology, a different approach to the mind was taken: it was believed that traits of caracters were located in various part of the brain. But the study of brain-behaviour relation would have to wait the end of the 20th century and the advent of MRI to really jump forward.
With neuropsychology and neuropsychotherapy, we see the first real scientific approach to localize function in the brain. While scientists have been long limited to study special cases involving people with brain damage, functional imagery has allowed to study thought process as they occur. In this approach, the split between conscious and unconscious isn't as important as in earlier psychological theory. What neuropsychology focusses on is the localization of a reaction within the brain, and the assessment of whether certains actions do work and alleviate the patients suffering.
It is based on hard scientific evidence to evaluate how therapy works. Even though the science is only in it's beginnings, it has already shown some fantastic promises in assisting the therapist. It is unlikely that in the near future we won't need the work of a hypnotherapist but it would be foolish to ignore this purely "physical" approach to the mind in our practice.
Theories are only useful as framework and guidelines to how we proceed to relieve the suffering of a patient. As a hypno-psychotherapist, we must adapt these frameworks to the perception that the patient has of their own mind and use their perception to help them deal with their pain. Understanding the patient's perception of their mind is the only way to use the right words to get it to work. In Erickson's vision of the mind, we need to utilize what the patient think and make our theory/knowledge of the mind adapt to it in order to alleviate the suffering.