"To think is an intellectual work": Good grammar?

Before you get to naming WHAT it is, this is an infinitive versus gerund issue.

Yes - you can use an infinitive as the subject of the sentence. But be aware: you sound like René Descartes.

Infinitives emphasize the POSSIBILITY or potential for something and sound more philosophical. 90% of the time, you will use a gerund as the subject or complement of a sentence.

And here, you are talking about the ACT of thinking, rather than 'man's capacity for thought'.

THEN - are you emphasizing that it is a cognitive activity; or that it takes labour, effort?

So - choose:

Thinking is an intellectual effort. (emphasizing 'intellectual', as opposed to all the other kinds of effort in life e.g. heavy lifting: physical effort

Thinking is intellectual effort. (emphasizing the effort entailed in deep contemplation of a matter).

Ya didn't think it wus so complicated, did yus.

i would use a semi colon or comma after - to think - for emphasis

however my grammar skills are often awry

they all have slightly different meanings also

i like the work one best - because it is also evocative of the arts and effort