AI researchers have been extremely successful at finding and proving specific areas of intelligence. But what they haven't found is the "silver bullet" that is governing all forms of intelligence. For example, intelligence isn't just symbolic logic, as AI researchers from the 1950's reasoned. It isn't just relational logic or optimization, as those from the 1970's and 1980's reasoned. It isn't just statistical reasoning, as those from the 90's and 00's have reasoned. And it isn't just the ability to process mass amounts of knowledge (data) as those today currently reason. Significant advances in any specific area of AI will cause people to think that they have found a victory of Neats over Scruffies [2] (for example, the currently pervasive view that Deep Learning is the silver bullet that can actually get us all the way to AGI), but then an AI Winter forms [3], and all of a sudden the Scruffies were right all along.
My opinion is that the Scruffies have always been right. The only valid Neat view is that true intelligence is really just mathematics, but mathematics is the superset of all algorithms ever. If you want some real philosophy on the subject from one of the most intelligent men that have ever lived, read up on Marvin Minsky.
[1] http://en.wikipedia.org/wiki/AI_effect