I've always phrased it as such:

"In a lane between players of equal skill, neither will be able to kill the other, excepting one making a mistake. I can go for kills, and count on my opponent to make mistakes, or I can farm, which relies purely on my own skill. If my opponent makes a mistake and I can kill them, then I shall take that opportunity, but I shall not rely on that chance, and nor shall I neglect the certainty of CS for the potential payoff of harassment."

Though to be fair, I did learn last-hitting on Annie, who essentially relies on the whole "Farm first, harass second" because of the mana refund on Q.