Although these words are sometimes used interchangeably, particularly in infinitive constructions (apt to fall, liable to fall, likely to fall),1,2 using them interchangeably obscures subtle yet important distinctions between them.
Of the three, it seems that apt is the one most often used in its proper sense—perhaps because, unlike liable and likely, which both stress a potential consequence and thus are often confused, apt simply stresses inherent tendency (eg, “Elmer is apt to be a bit unsteady on his feet”).3 Also, whereas liable and likely refer to consequences and thus most commonly to conditions that will become manifest in the future, apt usually refers to conditions manifest in the past or present.1 In addition, some authorities hold that apt is perhaps most commonly used when referring to persons, although it can refer to nonhuman or inanimate subjects as well.1
So far so good—but, as alluded to above, tossing liable and likely into the mix can muddy the waters a bit. Both stress degrees of potential, but liable is the weaker of the two, used to stress possibility (eg, “Because Elmer is apt to be a bit unsteady on his feet, he’s liable to fall”) rather than outright probability.1 It has sometimes been held that liable should be used only when the subject of the sentence would face unpleasant consequences from the action expressed by the verb (“Because Elmer is apt to be a bit unsteady on his feet, he’s liable to fall and break a hip”).2 Certainly, of the three words under discussion, liable is the one most often taken as indicating that a consequence might be unpleasant or disadvantageous.3
Compared with liable, likely is a stronger term, used to stress probability (eg, “Because Elmer is apt to be a bit unsteady on his feet, he’ll likely fall if the steps are icy”) rather than mere possibility.1 However, unlike apt, likely used alone stresses no particular tendency in the subject that would enhance the probability of the outcome; moreover, unlike liable, it need not suggest the potential for an unpleasant consequence (eg, “Although Elmer is apt to be a bit unsteady on his feet, he’ll likely not fall, even if the steps are icy”).2
The bottom line:
● Looking for a word that stresses inherent tendency, particularly in a person? Use apt.
● Looking for a word that stresses possibility (as opposed to probability)—especially the possibility of some unpleasant consequence? Use liable.
● Looking for a word that stresses probability (as opposed to possibility), whether the perceived consequences be good or ill? Use likely.—Phil Sefton, ELS
1. Apt, liable, likely. Merriam-Webster’s Dictionary of Synonyms. Springfield, MA; Merriam-Webster Inc; 1984:56.
2. Liable. TheFreeDictionary.com website. http://www.thefreedictionary.com/p/liable. Accessed March 8, 2012.
3. Apt, liable, likely. In: Bernstein TM. The Careful Writer: A Modern Guide to English Usage. New York, NY: Athaneum; 1985:48.