We can't isolate it and we can't examine it because it quickly converts into the products. So the activation energy, delta G with that symbol describes the amount of energy that must be supplied to any reaction in order to actually get it going. Now the activation energy describes how quickly a reaction actually takes place. So a reaction can be spontaneous, it can have a negative delta G value, but it can take place very, very slowly. And if a reaction takes place very slowly, what that means is it has a very high activation energy. So activation energy is not the same thing as Gibbs free energy.