A few things:

* Why is this called `aliased_method`?  Where is aliasing happening?
* `method_defined?` comes from [Module](http://ruby-doc.org/core-2.0.0/Module.html#method-i-method_defined-3F) and is not available on all objects.  I was going to ask how this is even working, and I think I realized it: your tests only get here with `Object` as the partial mock.  You should add some specs for a non-module partial mock and check if it works; I suspect it won't.
* The `method` method is public -- so why bother with `__send__`?
* For partial mocks, we already have the original method available via `original_method` -- any reason not to use that?
* It looks like this approach will only work if the method is defined and not if it is handled via `method_missing`, but if the partial mock claims it responds to the message (via `respond_to?`) we should count that as "good enough".