impractical argument; ivory-tower theory; student's naive reasoning
Refers to an argument or theory that is bookish, idealistic, or out of touch with reality, often associated with a young student's perspective. Slightly dated or literary in tone.
His proposal is nothing more than an impractical student argument and is useless in the field.
Even if you brandish such ivory-tower theories, it won't solve the real problem.
机上の空論 is a common phrase for armchair theory; 書生論 is less common and more explicitly tied to a student's naive viewpoint.
Compound of 書生 (shosei, 'student') and 論 (ron, 'argument/theory'). The exact historical derivation is uncertain, but the word likely emerged in the Meiji or Taisho period to describe the idealistic, impractical arguments of young students.