Keynes provided not one but two formulae for calculating the relationship between the Present and the Future. On the one hand we have the well-known Marginal Efficiency of Capital (MEC), where the value of the present stock equals the discounted sum of future earnings, but on the other we have the Conventional Projection (CP) where the expected earnings in the future reflect the productivity of present facilities. The MEC is likely to reflect the views of fund managers, the CP the views of corporate managers. These two sets of judgments of the future may coincide or diverge. Either way they determine the value of the present capital in terms of claims to it-and this determination may be stable or unstable, but in any case will be volatile in the sense that the underlying uncertainty can lead to rapid shifts. By contrast, the Classical Equations will give us the value of present capital in terms of its replacement or real cost. This is reliable, being based on the real side of the economy. The ratio of this to the uncertain and volatile MEC/CP financial calculation is analogous to Tobin's Q, and will play a role in determining investment decisions, but it chiefly gives us insight into how volatile those must be.