What is the difference between proper and common adjectives?

Proper adjectives have their initial letter capitalised, for example American - derived from a proper noun America - or Victorian - derived from (queen) Victoria.

Common adjectives, on the contrary, don't have their initial letter capitalised, for example green, empty, colourful. Just remember that "common adjective" is not a scientific term used by linguists, it is informal.

The Prince, by Niccolo Machiavelli, was written in 1513 in Florence, the city where he lived, at the time when Lorenzo de´ Medici ruled it and pope Julius II was head of the Catholic Church. Machiavelli did not believe in the principles of ruling set by both philosophy and Christianity, but rather supported principles of harshness and even brutality when it came for a prince or ruler to keep his/her power. In fact, he encouraged the rulers of his time, especially the Medici, to use any and all means for the sake of retaining their power, because this would mean order and peace for the then constantly warring states of the Italian peninsula. What is important to know about The Prince is that Machiavelli did not attempt to philosophize on ruling and ruling principles, but rather create a how-to guide for rulers and he tended to use real-life examples to reinforce his ideas. This is why, in the passage here, he mentions an example like Francesco Sforza, a real historical character. Because Machiavelli wants through historical facts and data reinforce his point on the correct principles that make up a prince especially in the arts of war and battle. The correct answer then is anecdotal evidence.