The second one is "chi = alpha plus four." The general point he is trying to make here is that in a Greek word we would see the chi and alpha because of the co

The second one is "chi = alpha plus four." The general point he is trying to make here is that in a Greek word we would see the chi and alpha because of the context, but if you typeset an equation with letters intended for setting text there simply isn't enough distinction in the characters to prevent confusion because there isn't enough context to distinguish the "a" and the alpha. You need a mathematical typeface with characters that cannot be confused even though text set with those characters would look awful. In other words, a proper mathematical typeface requires more than just symbols.