Inertia is mental, vital, physical, subconscient. Physical inertia can produce mental inertia, mental inertia can produce physical inertia, vital inertia almost always makes the physical lifeless and lustreless and dull, and that is inertia. Vital inertia can also infect the mind, unless the mind is very strong and clear. I have always said that the physical consciousness is the main seat and source of inertia.
Sri Aurobindo – Letters on Yoga: CWSA, Vol. 31, p539
It [the nature of the obstruction of the physical consciousness] depends on the weak points of the individual and the stage of his progress. In a general way, the obstruction creates an inertia which impedes the working of the higher Powers. In the early stage it can obstruct progress altogether. Afterwards it works to slow it down or else impede it by intervals of stationary inertia. The main difficulty of the physical consciousness is that it is incapable, before it is transformed, of maintaining any tension of tapasya—it wants periods of assimilation, sinking back into the ordinary consciousness to rest,—also there is a constant forgetfulness of what has been done etc.
Sri Aurobindo – Letters on Yoga: CWSA, Vol. 31, p390