  We discuss physical implications of the explicit method in numerical
analysis. Numerical methods have there own condition for causality, known as
the Courant-Friedrichs-Lewy condition. It is proposed that numerical causality
merges with physical causality as the grid interval size approaches zero. We
discuss the implications of this proposition on the numerical analysis of the
wave equation. We also show that, insisting on physical causality, the
numerical analysis of Schrodinger's equation implies that the minimum space
interval should satisfy $\Delta x \ge a_0 \lambda_c$, where $\lambda_c$ is the
reduced Compton wavelength and $a_0$ is a constant of the order unity.
