אור החיים — יערף כמטר לקחי תזל כטל אמרתי

Q — Moshe's song likens his teaching to both rain (מטר) and dew (טל) — 'My lesson shall drip like rain, my word shall flow like dew.' Why two different images, and what do they correspond to?

A — Or HaChaim explains that the first half ('לקחי' — rain) refers to the Written Torah, and the second ('אמרתי' — dew, 'matters transmitted mouth to mouth') refers to the Oral Torah. The Written Torah is compared to rain because it is 'the foundation and essence of everything'; the Oral Torah to dew because it provides 'the refinements and precise details not made explicit but only hinted at' in the written text. Just as dew completes and establishes what the rain has done, so the Oral Torah completes and establishes the Written Torah. (He adds that the comparison also obligates Israel to produce Torah from their mouths as steadily as rain falls — and that God does not demand Torah-study before providing sustenance.)