Java Reflection: Can't get to reach a class method

This is my first post here and I'll try to be short and specific.

I have the class Neuron, which contains a private field Class<? extends ActFunction> function; Whenever I create one of these Neurons, I pass to the constructor a subclass of ActFunction to be assigned to that field, all of the ActFunction subclasses have the method activate(double[] values), but when I try to invoke the method inside my Neuron by doing this:

Method activate = this.function.getClass().getMethod("activate");
return (double) activate.invoke(this, inputs);

it says that the method couldn't be found.

What am I doing wrong here? I'm not really an expert of reflection so I guess I'm missing some basic knowledge here.