This is doubtless the force of 易, its opposite being 險. Thus, Tu
Mu explains it as 坦易平穩之處 “ground that is smooth and firm,” and
therefore adapted for cavalry; Chang Yü as 坦易無坎陷之處 “level
ground, free from depressions and hollows.” He adds later on
that although Sun Tzŭ is discussing flat country, there will
nevertheless be slight elevations and hillocks.
with rising ground to your right and on your rear,
The Yü Lan again reads 倍 for 背. Tu Mu quotes T‘ai Kung as saying:
“An army should have a stream or a marsh on its left, and a hill
or tumulus on its right.”
so that the danger may be in front, and safety lie behind.
Wang Hsi thinks that 後生 contradicts the saying 視生 in § 2, and
therefore suspects a mistake in the text.
So much for campaigning in flat country.