During the later stages of the bull market culminating in 1929, the public acquired a completely different attitude towards the investment merits of common stocks. Why did the investing public turn its attention from dividends, from asset values, and from average earnings and transfer it almost exclusively to the earnings trend, ie the changes in earnings expected in the future? The answer was, first, that the records of the past were proving to be an undependable guide to investment; and, second, that the rewards offered by the future had become irresistibly alluring.
Along with this idea as to what constituted the basis for common stock selection emerged a companion theory that stocks represented the most profitable and therefore the most desirable media for long-term investment. This gospel was based on a certain amount of research, showing that diversified lists of common stocks had regularly increased in value over stated intervals of time for many years past.
These statements sound innocent and plausible. Yet they concealed two theoretical weaknesses that could and did result in untold mischief. The first of these defects was that they abolished the fundamental distinction between investment and speculation. The second was that they ignored price of a stock in determining whether or not it was a desirable purchase.
The notion that the desirability of a common stock was entirely independent of its price seems incredibly absurd. Yet the new-era theory led directly to this thesis. An alluring corollary of this principle was that making money in the stock market was now the easiest thing in the world. It was only necessary to buy "good" stocks, regardless of price, and then let nature take her upward course. The results of such a doctrine could not fail to be tragic.
Graham and Dodd, Securities Analysis 1934