My current thought process:
I don't think there are any "physical principles" unique only to Aikido, but the manner in which the principles are combined with the physical movements is unique. For example, music consists of different tones (notes) and rythmic patterns, and the way in which the notes and rythym are combined creates something distinctive. A style, if you will... i.e. "classical", "jazz", "country", "rock and roll", etc. Each of those styles of music, while still containing notes, rythmic patterns, particular scales and so on, contain combinations of each of those elements that are unique unto themselves, creating a sound that can (usually) be readily identified as belonging to the particular genre.
Indulging myself here, the metaphor could be extended further: certain genres of music are more encompasing and complex than others. For example, classical music is generally considered more complex than say, blues or country; requiring a more in-depth understanding of music theory and practice (I realize this is a generalization)...just as some martial arts generally require a greater understanding of body mechanics, control of space and timing, and a larger syllabus of techniques (jiujitsu for example) while others are seemingly less complex (like many striking arts). I usually compare Aikido to Jazz. To play Jazz music well requires a very thorough grounding in basic music theory and a complete understanding of rythm, chords, scales (classical music theory and practice) before those things can be combined uniquely and played with real proficiency. To truly practice Aikido requires a very comprehensive understanding of physical principles of movement, timing, spacing, and body mechanics, etc before the ability to understand and apply the actual "aiki" are available to a practioner.
So, it begs the question: What makes Aikido, Aikido? There must be something physically distinctive about Aikido that differentiates it from other arts....isn't there? Maybe others can put what that is in better terms than I, so in the meantime here is what I say:
"I can't tell you what it is, but I know it when I see it" :