I take passive investing to mean you can't predict future returns, so you are best to try and just buy a market index and hold it. What I think is pretty much Bogle's view.

This would be all well and good if investment growth was additive. But it's not. It’s multiplicative.

Multiplicative means the volatility effects returns:

Realized Return = expected return -1/2 volatility ^2.

While expected returns are not really predicable, volatility is.

Therefore, when a passive investor holds their portfolio stead as volatility rises, they are making an active bet that the expected return also rises. In times of high volatility like March, this can imply an annual expected return of 40%+. Would a passive investor go for that?

In our world with multiplicative returns, today’s passive investing it is constantly making active bets on future returns. If you believe you can’t predict future returns, but that you can predict volatility, then you should be selling when volatility rises, and vice versa.

Maximizing geometric returns is the true way to invest without conviction on future returns. Its the true passive investment style.