Categories for the Working Mathematician
《Categories for the Working Mathematician》 · 第 70 页 · Chapter III, Section 3
A product of objects \(a\) and \(b\) is a universal pair of arrows \(p:a\times b\to a\) and \(q:a\times b\to b\): every pair \(f:c\to a\), \(g:c\to b\) factors uniquely through \(a\times b\).