Example : Contemplate one's navel.

Example : I contemplated leaving school and taking a full-time job.

Example : He is meditating in his study.

Synonyms : meditate, study

Example : I mulled over the events of the afternoon.
Philosophers have speculated on the question of God for thousands of years.
The scientist must stop to observe and start to excogitate.

Synonyms : chew over, excogitate, meditate, mull, mull over, muse, ponder, reflect, ruminate, speculate, think over