If you can, forget about the milky pop direction that Chicago took during the 1970's and concentrate on their first few years together, 1968-1970.  Those were the formative years when the band was experimenting with jazz rock sounds that few others were making.  Some of the output, today's track is a great example, were flat out rockers.  They gave Steve Winwood's I'm A Man such a blistering that it nearly seems theirs.  5:20pm this afternoon, on the radio or on-line.