Humans are born with a great deal of knowledge built into them by their DNA.(see Chomsky for example) This was obtained from hundreds of millions of years of learning (by evolution).
AI might be impractical if knowledge learned during an AI's "lifetime" turns out to be small compared to what has to be built in to the AI to begin with. (i.e., to even get it to BE intelligent)
Asa H 2.0 light (see my blogs of 10 Feb. 2011 and 14 May 2012) has a minimum size of 8-9 kB (depending if its coded in BASIC or C++). A casebase (knowledgebase) of about 50 simple cases has been learned by Asa H 2.0 light and has a size of about 10 kB.