EDUCATION

temporarily wholly engaged in thinking, either because one is thinking carefully about sth or considering a problem

I became vividly aware of this disturbing phenomenon while I was sitting deep in thought on Hammersmith Bridge this afternoon.  BNCI

 My father could be seen standing by those four stone steps, deep in thought. BNCI

I saw in my mind Boullée in his room in Paris, deep in thought, his eyes on the horizon --; not your horizon or mine but a far more distant one. BNCI