The oracle bone form is an eye drawn sideways, pupil and all. Somewhere along the way scribes turned it upright to fit the square writing block, which is why the modern 目 looks like a window rather than an eye. The two inner strokes are still the pupil.
Modern Chinese usually says 眼睛 for a physical eye and keeps 目 for the figurative: 目的 (purpose, 'eye-target'), 目前 (at present, 'before the eyes'), 节目 (programme, an item on a list). 一目了然 means to grasp something at a single glance.
An EYE tipped upright — the two lines inside are the pupil.
目 is an eye rotated 90 degrees. Turn it back on its side and the oracle bone drawing reappears.