The American stock market is on fire. The S&P 500 index, a benchmark for U.S. equities, has been printing fresh all-time highs this year and trading at a staggering 70% above its historical average when measured by the 10-year cyclically adjusted P/E ratio, known to many as the Shiller P/E Ratio.

The question on many investors' minds is whether this is the time to pull back on stock investments, particularly those tracking this benchmark index like the Vanguard S&P 500 ETF (NYSEMKT: VOO).

To answer this question, we must consider past lessons within the context of the unprecedented era humanity is entering.

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Source Fool.com