There are 6 sense organs, termed internal bases, which possess the property of enabling that Consciousness to arise into activity when they are impinged upon by an appropriate stimulus. They are the sense bases. Then, there’s the sense-objects, called external bases, which give to objects their innate properties of bringing the senses into activity when under appropriate conditions they impinge upon them.
They are called the object bases, namely, visible (visual) base, ear base, etc. The 6-sense bases consist of material qualities derived from the 4 Great Primaries or Essentials. These material qualities are of an extremely subtle and special nature, for it is by way of these internal bases and their contact with the external stimulus or object, that active Consciousness concerning the object is able to arise.
It’s a wondrous 6-sense Organism, which produces Consciousness of different kinds when a material thing or idea, called the object, comes in contact with a sense organ, which is another material thing. The 5th proposition of the Law of Dependent origination says that the six bases beget contact. Contact is the conjunction of the inner and outer bases to produce feeling, or vedanā, of the 6th proposition.