Cube Counting invisible cub

[drillbitjoe]

Advertisement - Members don't see this ad

I came across this one today on crack PAT...so it turns out there is no cube on the ground-level row in the middle. Since it is being blocked, I assumed there was one there due to "row continuation". How can you know there isnt?

 

[derbear]

One of the rules of cube counting:
1. The only hidden cubes are cubes required to support other cubes.