Speculative is not the opposite of rational. Speculation can be entirely rational.

Perhaps you meant 'emotional' or 'irrational' rather than 'speculative'.

I agree.

What I meant was that much investment is speculative, and that speculative investing is not solely, or even mostly, based on a purely rational model e.g. one that is based on a prediction of future cash flows.

It is usually based on betting on the the future price of the asset, independent of fundamentals. Some of these approaches are more justifiable than others: market momentum, qualatiative prediction of the company's valuation trajectory, trendy but questionable financial metrics, sophist technical analysis etc.