The story of the birth of accounting begins with numbers. In the 1400s, much of Europe was still using Roman numerals, and finding it really hard to easily add or subtract. (Try adding MCVI to XCIV.)
But fortunately, Arabic numerals (1, 2, 3, 4, etc.) started catching on, and with those numbers, merchants in Venice developed a revolutionary system we now call "double-entry" bookkeeping. This is how it works:
Every transaction gets entered twice in financial records.