Let's assume that there are equal bodies of evidence concerning the possible consequences of mass immigration -- that is to say, that there is as much information supporting the pro-immigration lobby's fear of economic stagnation and the failure of certain social welfare programs without mass immigration that there is supporting the anti-immigration lobby's fears of total social collapse and economic ruin with it. All else being equal, prudence dictates we ought to favor the anti-immigration lobby's arguments, for two reasons: (1) If their policies prove to be unworkable, they can always be reversed. (i.e., unseal the borders) (2) If the policies of the pro-immigration side prove unworkable, they cannot be easily reversed. That is to say, if we sealed the borders and it was later determined that mass immigration is good, the borders could always be opened again. But if we opened the borders and later determined that mass immigration is bad, it would be difficult, if not impossible, to deal with the continuing social consequences of it, as at that point, sealing the borders would not effectively solve the problem. Thus the precautionary principle (as well as mountains of historical and sociological evidence) argue in favor of immigration control.