To an economist or a journalist who is used to thinking of a decade as long enough for entire theories to rise and fall, an age is the longest period imaginable; durable ideas can be age-old and this age is synonymous with 'this world' or 'everything'.
Can anyone explain why thinking, and not think is used in that passage?
Would it be ungrammatical replacing thinking with think?
Or, otherwise, that replacing would be grammatical but it would produce a change in meaning. If so, can anyone explain what this change in meaning is?
Best Answer
A good trick to avoid such confusion is to remember that there are two different entries used to in a dictionary. The typical usages are:
Compare these two sentences:
The first (I am used to playing) means: I have played badminton often enough, so you can expect me to know what badminton is and to play it well enough. The second (I used to play) means: I played badminton regularly for a certain amount of time in the past. (I don't play it anymore.)
In your question, it would be ungrammatical if you replaced thinking with think. Do not write
"He is used to think ...". Always write "He is used to thinking ..." to mean that he is familiar with that kind of thinking. However, if you want to mean that he changed his opinion, use "He used to think ...". For example, "He used to think smoking is good."