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Via d2r I ran across this post discussing the type systems of Java and Ruby. Other than the example using a private method (Smalltalk has no such thing), you can apply the same comparisons to Smalltalk:

Java uses inheritance (loosely speaking) as the mechanism for defining the type of an object. An object "is a" X if it implements X. While Ruby also supports inheritance, establishing "is a" relationships is not its main purpose. In Ruby an object is of a certain type if it behaves as that type. Thus the saying "if it walks like a duck and quacks like a duck, then it must be a duck" or as it is commonly called, Duck typing. It must be noted, however, that while Ruby has classes, objects are not explicitly declared of to have a certain type.

So, what, concretely, defines what type an object is in Ruby? Well, that's the wrong question to ask. But if you'd still like to hear an answer, an object, conceptually, is of N! types (made of the subsets of all the combinations of methods exposed by the object), where N is the number of methods exposed by the the object,

In a sense, Ruby shifts the notion of type to the client (the client being the code that uses a given object). Thus, an object is (or not) of the type required by client if it implements the operations required by the client.

I like the term "duck typing" too - it's a nice shorthand for how dynamic systems work.