Putting aside the definition of Ninjutsu for a moment, the main question is how does one learn ninjutsu?
As Westerners studying this topic I feel that we are at a great disadvantage, one the needs to be realized and accounted for before we can even attempt to learn ninjutsu/ninpo.
Our entire learning model is at the exact opposite with how one “learns” ninjutsu- isshi soden. In my own personal training it took me a long time to undo this damage, so the soon a person, you, may realize and act on it the sooner off you will be better in your training and taijutsu skills.
In the Wet we learn according to a very academic model which is not only mass produced, but also rewards learned thinking. Read books, listen to professors, take tests, complete courses.
Complete enough of these, or put in twelve years and you get a high school diploma, B.A/S., M.A/S., or even more letters in a PH.D.
Learning also only takes place in sanctioned setting by the state- public school, private schools, or higher education institutes accredited by the middle states association.
Our minds are conditioned through academic culture, and what one needs to do to “pass” and “learn”.
We know nothing else, and are told by those guiding us that this is the way to learn.
So when one want’s to learn ninjutsu, how else would we look to learn it?
Ninjutsu is learned through the physical of taijutsu. This is a moving meditation, an understanding of the forces of nature to be found in distance, timing, rhythm- the interaction of human beings in that movement.
Shin Gi Tai.
A few of the dozen or so training “concepts” that we hear- ideas of movement frozen for a moment so we can experience them.
These philosophies that allow Ninjutsu to work can not be studied academically, they need to be encounter in the training, in the dojo, the teacher creates them in the moment like flash for the student to experience. An experience that can not be understood or academically reproduced, but only felt in the moment.
You have to be there to be a part of it.
We also hear about nature and how it is important in understanding ninjutsu. Nature is dealing with the day to day interaction between yourself, others, and what fate throws at you. Holding these philosophies learned through movement and expression in the dojo and using them to navigate the day to day with a sincere heart and without even thinking about it.
The entire world is the class-room and your learning is the navigation through it.
Ninjutsu is “easy” to learn in that no academic proficiency or talent is needed- show up and train with a good heart, let those moments of enlightenment in the dojo happen, feel them, taste them, and when you are confronted by the impossible outside of the dojo use those feeling to navigate them and win.