We know we can quantify intelligence as in IQ because it is what we call aptitude in several tasks (e.g. pattern recognition, short-term recall), and we've found that they are correlated, and aptitude in those tasks is measurable (e.g. ability to recognize pattern and time taken, ability to recall and time taken). If intelligence as in IQ wasn't as transferable as it is we would be calling them different things. For example, intelligence as in IQ and being knowledgeable are different aspects of the popular notion of being intelligent or smart.