Claude C. Hopkins never wrote a blog post. He didn’t need to optimize for mobile devices or decide whether to use Flash. He never ran a PPC campaign or designed an infographic.

Yet, at the turn of the 20th Century, he created a blueprint for the industry that still guides the decisions we make in today’s marketing arenas.

His first book, Scientific Advertising, is regarded by many as the advertiser’s bible, and it’s chock-full of valuable insight on reaching your audience and converting them. Here are a few highlights:

1. “Forget yourself entirely. Have in your mind a typical prospect, interested enough to read about your product. Keep that prospect before you. Seek in every word to increase your good impression.”

In other words, write to your target audience, not yourself. What sounds good to you might not be enough to move potential buyers, or worse, it might turn them off.

You might be thinking “the fastest on the market” has a good ring to it while your prospects are looking for hard numbers. Or maybe you’re talking about all of the awards your service has won when they’re more interested in what it can do for them.

One of the best ways to keep the focus on your prospects is to visualize just one of them. Picture a face, give her a name, and write to that one person.

Even if you’re just writing a Google ad, don’t worry about what sounds good or if the words “pop.” Instead ask yourself, “what 130 characters does this one prospect need to read for them to click through?”

2. “Almost any questions can be answered, cheaply, quickly and finally, by a test campaign. And that’s the way to answer them – not by arguments around a table.”

There was advertising before Hopkins, but he was one of the first to establish testing as a vital part of the process. Why? Because, as he goes on to say, “none of us know enough peoples’ desires to get an average viewpoint.”

Without testing, all we can do is guess what people want, what they’re willing to pay, whether they’ll buy again, whether there’s a big enough market and how difficult that market is to penetrate.

Hopkins recognized that there are way too many important factors to leave to speculation and that going back-and-forth with guesses and opinions is a waste of time. He decided to “let the thousands decide what the millions will do” by running his campaigns through a few small towns before going national.

In the digital age, testing has become even more vital because there’s more competition giving each target more options.

Luckily, it’s also cheaper and quicker. Unlike Hopkins, you have the power to analyze test data in real-time, instantly modify parameters if necessary, and stop the test as soon as you get the results you need. Techli has a good list of split-testing tools that make the process even easier.

3. “The only purpose of advertising is to make sales. It is profitable or unprofitable according to its actual sales. It is not for general effect. It is not to keep your name before the people. Treat it as a salesman. Force it to justify itself. Figure its cost and result.”

Sure, there’s a lot to be said about the importance of raising brand awareness, informing, entertaining, sparking a conversation, becoming a thought leader, and so on, but if all of that isn’t leading to sales in the end, how much does it really matter?

This is not to say you shouldn’t try for any of the above-mentioned goals, but you should always set out to achieve them with sales in mind.

Before you publish that ten-step guide, make sure it includes a path to conversion even if you’re not pitching your product directly in the text (e.g. a targeted call-to-action in the sidebar).

Content is king, but you’ll find yourself in check if you’re not tying it to sales and revenue.