The sense-mind is more of an instinctive and mechanical mental process, reacting to impressions and responding with habitual patterns. As the human individual evolves, however, we see the start of a self-aware, reflective consciousness which has the ability to distinguish its awareness from the action of the sense-mind, observe it and enforce changes to habitual responses. This brings about the development of the power of mentality called the Buddhi. This is a power that uniquely identifies the human being from the rest of the animal kingdom.
Sri Aurobindo describes the Buddhi: “From the point of view of Yogic knowledge we may say that it is that instrument of the soul, of the inner conscious being in nature, of the Purusha, by which it comes into some kind of conscious and ordered possession both of itself and its surroundings. Behind all the action of the Chitta and Manas there is this soul, this Purusha; but in the lower forms of life it is mostly subconscient, asleep or half-awake, absorbed in the mechanical action of Nature; but it becomes more and more awake and comes more and more forward as it rises in the scale of life. By the activity of the Buddhi it begins the process of an entire awakening. In the lower actions of the mind the soul suffers Nature rather than possesses her; for it is there entirely a slave to the mechanism which has brought it into conscious embodied experience. But in the Buddhi we get to something, still a natural instrumentation, by which yet Nature seems to be helping and arming the Purusha to understand, possess and master her.”
Sri Aurobindo, The Synthesis of Yoga, Part Four: The Yoga of Self-Perfection, Chapter 7, Purification–Intelligence and Will, pg. 638