This is a quote from Shelby Foote in the PBS Civil War documentary by Ken Burns. I consider him to have been the foremost authority on the Civil War, and his three volume history of it is perhaps the most thorough work on the subject.
I agree with him completely; this is a true statement. The South was outnumbered 4 1/2 to 1 in terms of white population, dominated in industrial production and railroad capacity by the North, and the Union retained the entire US Navy during the war which was key for the blockade. These were simply too many disadvantages to overcome.
The South was playing defense, which was easier,and they had much better military leadership, but this was not enough. The only real chances they had at winning the war was if a foreign country like Britain intervened on their behalf and lifted the blockade, or if McClellan had won the Election of 1864 against Lincoln. Those were both longshots though.
I agree with the previous post. The South could not match the manpower or resources of the North. The North had more factories, money, people, and railroads than the South. The major advantage of the South was the military leaders. Eventually, these disadvantages were too much for the South to overcome.