ommitting murder. Two, murder may be defined as an act of violence,
consisting of breaking, mangling, maltreating or otherwise stopping the
functions of a living organism by a living organism. Three, most
murderers are detectable by certain chemical and electrical changes."

Macintyre paused to light another cigarette. "Those conditions take care
of the routine functions. Then, for the learning circuits, there are two
more conditions. Four, there are some living organisms who commit murder
without the signs mentioned in three. Five, these can be detected by
data applicable to condition two."

"I see," Gelsen said.

"You realize how foolproof it is?"

"I suppose so." Gelsen hesitated a moment. "I guess that's all."

"Right," the engineer said, and left.

Gelsen thought for a few moments. There _couldn't_ be anything wrong
with the watchbirds.

"Send in the reports," he said into the intercom.

* * * * *

High above the lighted buildings of the city, the watchbird soared. It
was dark, but in the distance the watchbird could see another, and
another beyond that. For this was a large city.

To prevent murder ...

There was more to watch for now. New information had crossed the
invisible network that connected all watchbirds. New data, new ways of
detecting the violence of murder.

There! The edge of a sensation! Two watchbirds dipped simultaneously.
One had received the scent a fraction of a second before the other. He
continued down while the other resumed monitoring.

_Condition four, there are some living organisms who commit murder
without the signs mentioned in condition three._

Through his new information, the watchbird knew by extrapolation that
this organism was bent on murder, even though the characteristic
chemical and electrical smells were absent.

The watchbird, all senses acute, closed in on the organism. He found
what he wanted, and dived.

Roger Greco leaned against a building, his hands in his pockets. In his
left hand was the cool butt of a .45. Greco waited patiently.

He wasn't thinking of anything in particular, just relaxing against a
building, waiting for a man. Greco didn't know why the man was to be
killed. He didn't care. Greco's lack of curiosity was part of his value.
The other part was his skill.

One bullet, neatly placed in the head of a man he didn't know. It didn't
excite him or sicken him. It was a job, just like anything else. You
kill