It happened around 7 p.m. Monday along the 4600 block of Cottman Avenue.
The victim's sister stopped by the scene Tuesday morning on her way to hospital and was visibly upset.
She asked not to be identified since police have not made an arrest. She said she doesn't know why her brother was stabbed.
"He's not a confrontational person. I have no idea. But when he wakes up we will sure find out," she said.
Witnesses tell police the victim had an altercation with another man. The man then stabbed the victim in the arm.
"I come outside. I see the blood and I'm like freaking out. I'm scared. I don't know what to do," said the victim's son David Finnegan.
"Apparently, causing a very severe injury probably hitting his brachial artery because he was bleeding profusely," said Philadelphia Police Commissioner Richard Ross.
"They cut down to the bone, To his tendons, through everything," said the victim's sister.
A witness called police.
Officer Maryanne Bennett arrived minutes later and immediately did something that likely saved the man's life.
"She immediately applied a tourniquet," said Commissioner Ross. "I don't think he would've had any chance of survival, according to witnesses at the scene he'd already started to lose consciousness."
That same officer transported the victim to the hospital where he remains in critical condition.
Family members say he's lucky to be alive.
Police have not made an arrest in this case and have not released a suspect description.