Jacob Lewis runs The Gym in San Bernardino County, and says he's decided to defy state shutdown orders because he believes gyms are "'essential' to people’s mental and physical health."

He opened his doors Friday morning at 6 AM, and several of The Gym's members are back inside getting their pump on, and sharing workout posts on social media.

Lewis tells TMZ, he tipped off cops and local officials to his plan to reopen, and received a summons from a code enforcement official. He's blowing that off, and says he doesn't anticipate any serious trouble.

He says equipment is spaced out in order to follow social distancing guidelines, and there's disinfectant everywhere. He says the showers and saunas are closed as a precaution, adding ... "Basically right now it’s a gym with bathrooms."

Lewis says he came to the decision to reopen after a recent conversation with an older man, who confided in him that The Gym is his "home away from home," and helps him stay healthy and battle diabetes. The man told Lewis he's falling apart without the facility.

That's when Lewis had his epiphany ... "I realized I was being a BITCH!! By letting down the people that matter most to our community!! By not providing a service that helps them get through all the bull s**t! And gives them hope, strength, and health."