Jun Fan Jeet Kune Do may be considered more of a science than a traditional martial art.

The core philosophy and techniques were distilled by Bruce Lee using a process that that closely replicates the “scientific method”;
1 Careful observation(s)
2 Deduction of natural laws [principles]
3 Formation of hypotheses
4 Experimental or observational testing of the validity of the predictions (hypotheses/principles) thus made.

Lee focused his observations on combat movement.

THEN (circa 1967):

 

Careful observation;
It is well know that Lee had carefully studied the science of biomechanics. His extensive library contained a number of kinesiology/biomechanics text books.
Bruce also read hundreds of books on different martial arts from Western boxing to traditional Karate and Gung Fu. He evaluated his own background and knowledge in Chinese martial arts (perhaps highly influenced by Wing Chun, although many of his first generation students would debate this).

He studied and analyzed boxing films of the great boxers of that era (such as Muhammed Ali (a technique also used in the study of biomechanics). 

Deduction of natural laws/principles;
He analyzed and then deduced fundamental principles of motion and force in combat and the “reality of fighting”. Lee asked himself questions that he continually refined.

Experimental or observational testing of the validity of the hypotheses and principles;
Sifu Lee constantly “tested” his hypotheses and applied his principles to what worked in realistic combat. Once he evaluated these through experimentation, he discarded what was “useless”. He continued this process of refinement and discarding remaining objective and unbiased as he “chiseled away at the unessentials”. A scientist.

He named his process of discovery Jeet Kune Do.

If one chooses to call Jun Fan Jeet Kune Do a martial art, then the emphasis must be on the martial (fight science) and not on the art.