Jung described each of the mental processes in terms of their attitudes. It wasn't just Sensing, it was Sensing in the extraverted attitude or in the introverted attitude. Functions used in the extraverted attitude have a here and now quality to them. Thus both extraverted Sensing and extraverted iNtuiting are in-the-moment perceptions. When we engage in either of them, our energy goes to either seeking more sensory input (Se) or to interacting to develop more ideas (Ne). In both processes, the focus is on the possibilities and opportunities. With extraverted Sensing(Se), the focus is on the immediate, sensory possibilities and options for action. With extraverted iNtuiting(Ne), it is on the envisioned possibilities, new ideas and meanings.
Functions used in the introverted attitude have a past, future or universal quality to them. Introverted Sensing has a past and sometimes universal sense with the focus on the evoked impression, usually of something perceived beforehand, or as Jung said, "...it would also see what was before their becoming and will be after their passing ..." (Psychological Types, p.395) Introverted iNtuiting is a process of becoming aware of what will be that hasn't yet been. Vision, foresight, and profoundly impactful symbols are often a result of this process.