I. It has been shown what the rules are, which ought to be observed in adjusting the proportion between the punishments and the offense. The properties to be given to a lot of punishment, in every instance, will of course be such as it stands in need of, in order to be capable of being applied, in conformity to those rules: the quality will be regulated by the quantity.
II. The first of those rules, we may remember, was, that the quantity of punishment must not be less, in any case, than what is sufficient to outweigh the profit of the offence: since, as often as it is less, the whole lot (unless by accident the deficiency should be supplied from some of the other sanctions) is thrown away: it is inefficacious. The fifth was, that the punishment ought in no case to be more than what is required by the several other rules: since, if it be, all that is above that quantity is needless. The fourth was, that the punishment should be adjusted in such manner to each individual offence, that every part of the mischief of that offence may have a penalty (that is, a tutelary motive) to encounter it: otherwise, with respect to so much of the offense as has not a penalty to correspond to it, it is as if there were no punishment in the case. Now to none of those rules can a lot of punishment be conformable, unless, for every variation in point of quantity, in the mischief of the species of offense to which it is annexed, such lot of punishment admits of a correspondent variation. To prove this, let the profit of the offence admit of a multitude of degrees. Suppose it, then, at any one of these degrees: if the punishment be less than what is suitable to that degree, it will be inefficacious; it will be so much thrown away: if it be more, as far as the difference extends, it will be needless; it will therefore be thrown away also in that case. The first property, therefore, that ought to be given to a lot of punishment, is that of being variable in point of quantity, in conformity to every variation which can take place in either the profit or mischief of the offense. This property might, perhaps, be termed, in a single word, variability.
III. A second property, intimately connected with the former, may be styled equability. It will avail but little, that a mode of punishment (proper in all other respects) has been established by the legislator; and that capable of being screwed up or let down to any degree that can be required; if, after all, whatever degree of it be pitched upon, that same degree shall be liable, according to circumstances, to produce a very heavy degree of pain, or a very slight one, or even none at all. In this case, as in the former, if circumstances happen one way, there will be a great deal of pain produced which will be needless: if the other way, there will be no pain at all applied, or none that will be efficacious. A punishment, when liable to this irregularity, may be styled an unequable one: when free from it, an equable one. The quantity of pain produced by the punishment will, it is true, depend in a considerable degree upon circumstances distinct from the nature of the punishment itself: upon the condition which the offender is in, with respect to the circumstances by which a man's sensibility is liable to be influenced. But the influence of these very circumstances will in many cases be reciprocally influenced by the nature of the punishment: in other words, the pain which is produced by any mode of punishment, will be the joint effect of the punishment which is applied to him, and the circumstances in which he is exposed to it. Now there are some punishments, of which the effect may be liable to undergo a greater alteration by the influence of such foreign circumstances, than the effect of other punishments is liable to undergo. So far, then, as this is the case, equability or unequability may be regarded as properties belonging to the punishment itself.
IV. An example of a mode of punishment which is apt to be unequable, is that of banishment, when the locus a quo (or place the party is banished from) is some determinate place appointed by the law, which perhaps the offender cares not whether he ever see or no. This is also the case with pecuniary, or quasi-pecuniary punishment, when it respects some particular species of property, which the offender may have been possessed of, or not, as it may happen. All these punishments may be split down into parcels, and measured out with the utmost nicety: being divisible by time, at least, if by nothing else. They are not, therefore, any of them defective in point of variability: and yet, in many cases, this defect in point of equability may make them as unfit for use as if they were.