Spenser for hire

Vaiva, an avid aficionado of mystery books, introduced me to Robert B. Parker's Spenser series.  Spenser is a tough-guy detective, Susan Silverman is his brainy girlfriend, Hawk his powerful sidekick.  The books are written in the first person, from Spenser's point of view.  We also enjoyed the TV show developed from the book, Spenser: For Hire, but the books were better.

I've been listening to another in the series, Rough Weather.  Spenser witnesses a murder at a wedding, the murderer is Rugar, someone he knows from earlier books.  The story is completely impossible, but still interesting.  At one point Rugar drops in on Spenser and Susan at a restaurant, to warn Spenser to drop his investigation.  Rugar solemnly promises that he won't harm Susan, but worries that he may have to kill Spenser.  Spenser accepts those terms.

After Rugar leaves them, I was struck by the following passage:

"Is his word, in fact, good?" Susan said.

"Yes," I said. "He won't bother you."

"You know that."

"Yes."

"I'm not asking for reassurance," Susan said.  "I believe you.  But how do you know?"

"Rugar's a professional killer, pretty much willing to do anything.  Unless he has some rules for himself, he has no limits, and he's in free float. There's no tether."

"So he makes some up."

"Yep."

"Do you do that?"

"Don't need to," I said.  "I have you."

I have no tether.