On Rubber Sheets
Recall, if you will, the rubber sheet used to explain gravity. This sheet is stretched taut, and objects – representing cosmic bodies, usually – are placed onto the sheet, creating an indentation proportionate to their mass. That is, small objects make almost no dent at all, while large objects create an indentation that stretches well beyond their own borders.
Now, translate that analogy to your more general awareness of the world around you. It's not a huge leap – we even speak of things "weighing on our minds." Of course, there's no longer a single measurable metric by which we gauge impact, but there is nonetheless variable weight imposed by different objects, in proportion to the effect they have on our lives, and on our awareness.
Now, much like you need to be aware of gravitational bodies in order to successfully navigate space, you need to be aware of anything that affects your metaphorical trajectory if you are to successfully navigate life.
Of course, no-one is aware of everything, so no-one navigates perfectly. If you're aware of this fact, you'll try to increase your awareness, and so increase the accuracy of your navigation.
Eventually, when you've increased your awareness sufficiently, you become aware of a shadow; a gravitational body which seems to have a significant and ever-present influence, but which you cannot see. Eventually, you realise that the shadow is the shadow of your own presence.
This is sentience.