What does it mean that capacity is finite even if ambition is infinite?

Accepting your actual capacity is the first step to using it well.

Example: You have forty-seven items on your to-do list, twelve active projects across three domains, and a calendar packed edge-to-edge from 7 a.m. to 10 p.m. You wake up energized, convinced that today is the day you finally catch up. By 11 a.m. you have attended two meetings, answered thirty emails, and made meaningful progress on exactly zero of the twelve projects. By 3 p.m. your brain feels like wet concrete. You push through anyway, producing work you know you will redo tomorrow. By evening you add three new items to the list and remove none. You do not have a motivation problem. You have a capacity problem — you have committed to roughly three times more output than any human nervous system can produce in a day, and no amount of willpower closes that gap.

Try this: Open your calendar and task list right now. Count every commitment you have made for this week — meetings, deadlines, projects with expected deliverables, personal obligations. Write the total number down. Now estimate your actual deep-work hours for the week: take your waking hours, subtract meetings, email, meals, commute, transition time, and recovery. Write that number down. Divide commitments by available deep-work hours. If the ratio exceeds 1:1 — if you have more commitment-hours than capacity-hours — you have just quantified your capacity deficit. Do not try to fix it yet. Just look at the number. The number is the lesson.