Proposed definition #1: You need a map
Introverted Sensation (Si) is the attitude that what is manifest (apparent, observable) is overwhelming in its complexity and patternlessness, and that the only way we can find our way through it is with a map. To make our way through the unpredictable, we need to anchor ourselves in some way, to know what, in that vast chaos, to focus on. (See Orienting.)
Proposed definition #2: The unknown is dangerous, so anchor yourself in the known
Introverted Sensation (Si) is the attitude that the unknown mostly contains threats that will undermine life and order. Given the precariousness of life--so many things have to be jjjjust right--the odds of something unknown being beneficial are very low. Consequently life demands that we carefully filter the unknown before letting it into a position of influence, that we construct barriers against the unknown, etc.
For example, in engineering, one is primarily concerned with designing systems that won't fail even though most of the exact causes of failure are not knowable in any precise way. Engineers learn many different ways in which things fail, and learn to design so that the things work, or at least major disasters don't occur, even when things go wrong--as they inevitably will. A bridge is typically designed to hold a load 6 times bigger than the biggest anticipated load, simply so it will resist unexpected troubles--shearing winds, or cracks in unexpected places, or who-knows-what that might come along and can never be fully anticipated. The full breadth of relevant dangerous factors is inherently unknowable.
An everyday example is to allow some extra time when leaving on a trip. The reason for leaving some extra time is because the world is filled with unpredictable things that could make you late. Very few unpredictable things could come along and make you early if you left late.
From the Si standpoint, the more you're going to depend on something, the more carefully you'd better inspect it, because you never know what unknown things might go awry, you only know that most of them are bad.