Chapter 10 of copywriting classic “Scientific Advertising” is titled “Things Too Costly”. By this, the great Claude Hopkins means…

“Many things are possible in advertising which are too costly to attempt. That is another reason why every project and method should be weighed and determined by a known scale of cost and result.”

Put another way, there are some projects that will never make money for you, no matter how good the copywriting and how worthy the project.

Here are some of the examples Hopkins mentions…
1. Trying to get people to change long established customs and habits is usually futile.

2. Trying to educate people is usually non-productive for an individual advertiser (and any success is usually shared with other businesses).

3. Selling “prevention” is a hard task.

4. The “economics” simply doesn’t work for many products.  To elaborate, the revenue per customer might simply not be enough to cover the cost of acquiring the customer (even if there’s a potentially big market for the product).

5. Some products might have 2 possible appeals, one to prevent a “negative” the other a “positive”.  Almost always, the “positive” appeal will be more successful.

In this last case, Hopkins provides a couple of examples including…

“A tooth paste may tend to prevent decay.  It may also beautify the teeth.  Tests will probably show the latter appeal is many times as strong as the former.”

As regards the problem of educating people about new products, Hopkins’ advice was to monitor popular trends and when the desire has been created by, for example, the mass media then step in and satisfy that desire.

Looking through these points, every one of them is still relevant today for advertisers.  Pretty much every copywriter has it drummed into them that prevention is a difficult sell and that trying to educate consumers is a costly undertaking.  This is the original source of that advice.

And it all came from hard lessons learned from testing…

“This chapter, like every chapter, points out a very important reason for knowing your results.  Scientific advertising is impossible without that. So is safe advertising. So is maximum profit.”