Marie-François-Pierre-Gonthier Maine de Biran - or as he is usually called, Maine de Biran - counted himself in the so-called ideological tradition of philosophy, continuing and building upon the works of Condillac and Cabanis - at least in his first major work, Mémoires sur l'influence de l'habitude, published in two parts. The word “ideology” does not refer to any political set of opinions, but to the Lockean habit of calling the basic elements of human consciousness ideas.
Although Maine de Biran identifies his work with the tradition beginning with Locke and Condillac, he is also extremely critical of the somewhat speculative manner in which the two gentlemen carried out their project. Instead, he is eager to follow Cabanis in his attempt to ground human mentality in concrete physiological studies. Thus, his first task is to differentiate between sensations and perceptions in physiological terms and not just by defining perceptions as a clearer form of perceptions. In fact, Maine de Biran appears at times even to suggest that when sensations have more clarity, perceptions are less clear, and vice versa.
In the first book, Maine de Biran suggests a quite concrete physiological difference between sensations and perceptions, sensation being an affection of sensory organ, while perception is formed by brain acting upon such affections. In the second book, the difference is laid out in more abstract terms - while sensations are passive, perceptions are active, just like nervous system consists of affective and motive nerves. In effect, Maine de Biran is giving a new physiological twist on the old idea of the difference between cognition and volition.
Since sensations and perceptions are two quite different and even opposed states of human mind, it is quite easy to see why making one stronger will weaken the other - if we are overwhelmed by strong sensory effects, we are far more likely to lose our ability to perceive the objects around us clearly. Then again, when the force of new sensations subsides, for instance, when we become accustomed to them, it is far easier to e.g. distinguish objects around us.
The example above shows us also an example of the supposed main topic of the book - the influence of habit. Indeed, it is just this habit of having sensations that allows us to have more control over our perceptions. This habit makes it easier for us to decide what aspects of our sensory field do we want to concentrate on, just like practicing any activity will make that activity easier for us.
Yet, Maine de Biran continues, habit is not just a positive force. This is especially the case with our imagination fathoming combinations between two quite distinct things: for instance, whenever we’ve seen one event following another, we easily conclude that one is the cause of the other, although there wouldn’t actually be any connection between them.
If imagination is involuntary and passive association of our ideas, the corresponding active movement from one idea to another Maine de Biran calls memory. Memory, he says, is essentially based on taking some sounds or inscriptions as signs of our ideas, and through their means actively recollecting the ideas associated with them. The recollection might stop at the level of mere signs, and then we are dealing with mechanic memory, that is, with mere repetition of a string of signs or words, known by heart. Then again, words might recollect, not ideas, but mere unclear feelings, and then we are dealing with sensitive memory - this is the faculty of poets and mystic philosophers.
The perfect mode of memory or representative memory works in many cases in close cooperation with the two other modes of memory. For instance, Maine de Biran points out that when we use words from ethics, such as good or virtue, we do attach to these words clear representations, but also certain affective emotions. Further complications arise from the fact that the representative memory has to sometimes deal with abstractions like time and space, which refer to a number of ideas.
A further level of habituation generates judgements - that is, a judgement is born when we frequently associate some ideas and signs together. If this association is made quite passively, the judgement is mechanical - like when we repeat the multiplication table by rote - but when we actually follow the train of ideas and signs we are dealing with a reflective judgement. Finally, the effect of habituation on judgements might again be either passive and harmful or active and beneficial. We might associate judgements which have nothing to do with one another, which leads to faulty reasoning. It is then no wonder that Maine de Biran concludes with the thought of representing all judgements in a Leibnizian-style universal calcul, which would allow perfect reasoning, like in mathematics.