In a recent essay, GMO co-founder and chief investment strategist Jeremy Grantham offered insights on the market by “looking very hard at all the great bubbles of the past,” and “searching for useful guides to the future.” His commentary was excerpted in Barron’s.

While Grantham agrees that today’s market is priced “exceptionally high,” he says the typical examples of past bubbles are “not just characterized by higher-than-average prices.” Indicators of “extremes of euphoria” he says, “seem much more important than price.” He underscores his belief (one shared by Robert Shiller) that the market is not showing enough signs of euphoria to “make this look like a late-stage bubble.”

Psychological factors, says Grantham, are making it difficult to compare today’s market with previous eras, “so let’s look at what is missing in the way of psychological and technical signs of a late-stage bubble and what is beginning to fall into place.”