These are two very small words that are life altering in their differences. When I think about this in the context of my spiritual training, the word "faith" comes to mind, but not necessarily with positive connotations, at least not immediately.
Immediately, I say, "Faith is credulity."
With that thought processed through what is, ironically, a questioning "faithless" mind, I say, "Faith is knowing."
One feels very different from the other. Physically. The feeling that pervades all my senses when I think the first, is tension. The feeling with the latter is a very much milder but just as relaxing dilaudid, the unbelievably yummy opiate I had after a surgery. That feeling was almost enough to make me want to stay in the hospital with the dosage button taped down. It's what addicts seek. It's the sensation that all is well everywhere, and in addition to being found in mind altering substances, it's found in "is", but not if because "is" requires we alter our minds ourselves to shift our perception. That feeling, minus the opiates, requires that we look through the swarm of our own thoughts to see what already exists.
After all my talk about the color yellow haunting me, I found my way back to "is" in a chartreuse shade of it. The image below is not a video, but it moves. It is still. It is moving. It's all about perspective because everything Is, If you look for it. Look for the good stuff.