Claude C. Hopkins was not in the business of selling ads. He was in the business of making ads that sell.
And he feverently believed that ads should be tested, tested again, and then tested -- all in the effort to continue to improve revenue.
On advertising, he wrote:
It is not for general effect. It is not to keep your name before the people. It is not primarily to aid your other salesmen. Treat it as a salesman. Force it to justify itself. Compare it with other salesmen. Figure its cost and result. Accept no excuses which good salesmen do not make. Then you will not go far wrong.
While born in 1866, Hopkins was made for the digital era of optimization and analytics.
Many agencies continue to approach advertising as a painter would a blank canvas -- forgetting that the finished work is not the final product for any agency. There are goals to achieve.
(View more about the life and work of Claude C. Hopkins in our The Social Network of Advertising Icons. View all 10 profiles, including ones for Leo Burnett, John Hegarty, Lee Clow, and Bill Bernbach, by clicking here.)
Be inspired by the man who transformed the art of advertising into a science:
15 Quotes on Selling From Claude C. Hopkins
Advertising is salesmanship. Its principles are the principles of salesmanship. 
Advertising is multiplied salesmanship. It may appeal to thousands while the salesman talks to one. 
You are writing on a serious subject -- the subject of money-spending.
A vast amount of space is wasted in displaying names and pictures which tell no selling story. The tendency of modern advertising is to eliminate this waste.
Your object in all advertising is to buy new customers at a price which pays a profit.
Bear in mind that you are the seller. You are the one courting interest. Then don’t make it difficult to exhibit that interest. Don’t ask your prospects to pay for your selling efforts.
Do only that which wins the people you are after in the cheapest possible way.
One may gain attention by wearing a fool’s cap. But he would ruin his selling prospects.
It is certainly unwise to spend large sums on a dubious adventure.
Let us know the cost of our pride. Then, if our advertising fails to bring the wanted returns, let us go back to our model—a good mail order ad—and eliminate some of our waste.
No amount of experience can guide us correctly in even the majority of cases. That is why incaution is an advertising crime.
It is always a question of what we are willing to pay for our frivolities.
Don’t try to be amusing. Money spending is a serious matter. Don’t boast, for all people resent it. Don’t try to show off. Do just what you think a good salesman should do with a half-sold person before him.
No one reads ads for amusement, long or short. Consider them as prospects standing before you, seeking for information. Give them enough to get action.
Ads are not written to entertain. When they do, those entertainment seekers are little likely to be the people whom you want.
Sources:My Life in Advertising and Scientific Advertising