[jblow]

He would be able to see a circle the same way you can see a sphere. Do you feel like you can only see discs, not spheres?

[FPGAhacker]

How different would a 2d square look projected onto a 1d view, vs a circle?

[javajosh]

If you rotate a square, then the length will oscillate. A circle will remain fixed (but presumably would have surface detail to clue you in that it's rotating.)

[jobigoud]

If the object is lit at an angle the shading will be completely different, the edge of the square will create a harsh line.

[shezi]

Assuming you have "1d depth vision", ie computing distance to the eye using two eyes set some distance apart, you'd be able to tell them apart rather easily, in addition to shading and movement differences.

Just like our 2d eyes do in our 3d world.