The sheep in the field (Chisholm 1966/1977/1989). Imagine that you are standing outside a field. You see, within it, what looks exactly like a sheep. What belief instantly occurs to you? Among the many that could have done so, it happens to be the belief that there is a sheep in the field. And in fact you are right, because there is a sheep behind the hill in the middle of the field. You cannot see that sheep, though, and you have no direct evidence of its existence. Moreover, what you are seeing is a dog, disguised as a sheep. Hence, you have a well justified true belief that there is a sheep in the field. But is that belief knowledge?
The real problem with undocumented immigrant workers is that flouting the law has become the norm, which makes the job of terrorists and drug traffickers infinitely easier. The economic costs of terrorism can be very high and very real, quite apart from the otherwise positive economic impact of immigration. In order to separate the good from the bad, there is no substitute for a nationwide system that identifies all foreign persons present within the . It is not sufficient to identify visitors upon entry and exit; rather, all foreign visitors must be quickly documented.