Philosophy of Hapkido
If we agree that the true philosophy of a martial art must be based on and logical connection to its techniques, we can naturally admit that the philosophy of Hapkido shares most essential part with Taekwondo philosophy. And additionally, as Hapkido is also a traditional martial art of Korea as well as Taekwondo, we have technical and historical basis for research of their sameness; the ways of Samjae and Kang-Yu.
But we'd better to observe their difference first. Let me review several explanations about it.
Summing up them above, the principles of Hapkido can be explainend like this: its main principle can be said to be that of Hapki(Collection of inner power) and it is to be performed through three ways; water, circular motion and harmony or non-resistance. This scheme is all that was suggested so far.
But philosophy is not scheme; it should be a systematical consideration of all parts of martial art. Additionally the principles are suggested too simply way, so they became dogmas. They need some comments. You can find them in PPT and 3JKY.
The principle of water is explained in Ch.23
And I'd like to point out that harmony is broader concept than non-resistance, so that they cannot be replaced or compared in the same level. Non-resistance is more concrete principle that can be applied to a technique while harmony is rather a sort of ideology. It is quite obscure to apply it to technique. The philosophy of Taekwondo tells us the difference.
In conclusion, Hapkido is another name of TAEKWONDO in its essential part. Thus, the philosophy of Hapkido is almost same as that of Taekwondo. In order to find some uniqueness of Hapkido, we can say that Hapkido is more soft and skillful system of offense and defense than Taekwondo. In that sense, Hapkido can be said to be more elegant than Taekwondo. Instead, it can be said its basis is TAEKWONDO.