Although an actual definition does not exist as to what songs can comprise a greatest hits collection, it is generally assumed that the disc will contain the biggest and best-charting singles issued by an artist. So it may seem odd for Rush to release a greatest hits collection when the group has had just one Top 40 pop singles' hit in their long career; but if you factor in the amount of rock radio airplay given to these songs, then The Spirit of Radio contains, in essence, the greatest hits of Rush.