We know what we are, but know not what we may be.
There's a subtle comfort here that most people miss—Shakespeare isn't urging you to dream bigger or reach higher, but rather acknowledging that our self-knowledge is permanently incomplete, which means judgment (especially self-judgment) must remain humble. The real power lies in recognizing the gap itself as liberating rather than frustrating; a person who feels trapped by past mistakes or present limitations has forgotten that the future contains possibilities their current understanding literally cannot access. Think of someone who left school at sixteen convinced they weren't an academic, only to discover at forty that they could master difficult subjects when the subject mattered to them—not because they changed fundamentally, but because the conditions for knowing themselves had finally shifted. That's what Shakespeare means: we're works in progress who can't read ahead in our own story.