Read the Solidity documentation on visibility [1]. Public, private, internal, external, that really seem to mix overlapping concerns. In the visibility documentation, it explains that the default is public.

Then, in another section of the documentation, when talking about function types [2], not specifying internal or external defaults visibility to internal.

It doesn't seem like the simplest or most consistent way to express these concepts, and certainly not the safest way.

[1] http://solidity.readthedocs.io/en/develop/contracts.html#vis... [2] http://solidity.readthedocs.io/en/develop/types.html#functio...