I say it is odd, because one might suppose that there is little difference between an image in two dimensions being used in devotion, and an image in three dimensions. Put in this way, as the bald numeric addition of an extra dimension, it does indeed seem arbitrary. But if one pays attention to effect on one's mind and heart and nervous system of an icon and a statue in a church, perhaps there is quite a profound difference between the two.
The Orthodox, while not banning the use of statues outright, have reservations about their use: St Nicodemus of the Holy Mountain says that icons are more pious and decent. There are at least four or five reasons commonly given for the reservation:
- A statue can be walked around and therefore treated as an object in its own right; it is inviting us to artistic appreciation rather than a face-to-face relation with a person. There seems to be two distinct notions here (1) that a holy image if venerated should not be objectified, for it is in some sense assimilated to the one whom it presents and (2) the related point that meditation upon an icon is a vehicle of spiritual communion with the person presented.
- Because a statue is three dimensional it becomes a part of the space it inhabits; it is bounded and limited by the walls, floor and ceiling. On the other hand, a two dimensional holy image gains something by its loss of spatial depth: it becomes a window into heavenly space. When one removes the dimension of space one is also removing its limits; for the loss of the physical, there is an augmentation of the spiritual.
- One doesn't know how or where to venerate them - does one kiss the foot, the knee? (Icons are venerated by kissing the hands or the feet of the person, or the bottom corner.) I suppose that there is actually a customary veneration that has attached itself to certain statues, for example the kissing of the foot of St. Peter in Rome's greatest basilica - but no general rule.
- Statues are too much like replications of their originals, which is not what a holy image is for: an icon should not even be portraiture. The flatness of the icon witnesses to its unlikeness to its original as much as its likeness, adding this "distance" from pictorial art to the traditions of non-literal depiction such as simplification of bodily form, elongated figures, and large eyes.
- Where a statue can be used for veneration - according to variations in local custom - for all of the above reasons it had better be placed against a wall or in a niche and be a fixed part of the Church and liturgical drama rather than a detachable art piece. Further, it should be in some way a stylised representation that aims to encourage mental vision - sculptures of the Anglo-Saxon, Carolingian or Romanesque periods are therefore preferred over the late-Gothic or Renaissance virtuoso performances.