Balance
Again every picture is subjective yet there is always a balance when looking at a really good image, one that makes the image memorable and pleasing to the eye. Balance dosen't mean that everything in the picture has to be in the centre quite the opposite infact, the eye takes in the camera viewpoint, tone, colour, framing and detail.
For example in this photograph the figure crossing the bridge has been captured just as they pass under the middle of the building creating a strong vertical line however either side of the figure the green colour tone balances the picture, also the red light right at the top of the picture helps the eye travel upward.
Finally sometimes shapes and proportions are objects in their own right, sometimes how they are cropped creates balance.
Next time "The rule of thirds".