Consider the following:
（ は / が ）食べないでください。
Person A: どちらが東ですか。
Person B： こちら
（ は / が ）東です。
Workbook answers are
はfor 1. and
In 1., I would expect the person to be pointing at the apple or at least gesturing with body language towards the apple. Furthermore the apple is physically present within proximity of the speaker as evidenced by use of "
この". Therefore the apple is already present in the "universe of discourse" and the use of は to extract it from the "universe of discourse" is justified.
In 2., Discourse is introduced by person A. The workbook chooses
がas the correct answer.
(Observations) The apple is tangible while direction is intangible. However, both are present in the "universe of discourse". "Direction" is part of "immediate environment" and I should always be able to refer to immediate environment using
は since it's shared information between two parties.
(Question) Can't I similarly take "direction" as already present in the "universe of discourse" and use は to refer to it in 2.?