Read the passage from The History of the Peloponnesian War by Thucydides. There, far from exercising a jealous surveillance over each other, we do not feel called upon to be angry with our neighbour for doing what he likes, or even to indulge in those injurious looks which cannot fail to be offensive, although they inflict no positive penalty. But all this ease in our private relations does not make us lawless as citizens. Against this fear is our chief safeguard, teaching us to obey the magistrates and the laws, particularly such as regard the protection of the injured, whether they are actually on the statute book, or belong to that code which, although unwritten, yet cannot be broken without acknowledged disgrace.

–The History of the Peloponnesian War, Thucydides

Why does Thucydides say that Greeks citizens are not lawless?

Greeks give one another “injurious looks” to enforce laws.
Greeks rely on a code of written laws and unwritten laws to not be broken.
Greek citizens spy on one another, making it hard for them to break the law.
Greeks behave similarly, so there is no reason to be concerned that someone will break the law.