Alone - (Webster's 1828 Dictionary)
ALO'NE, a. [all and one.]
1. Single; solitary; without the presence of another; applied to a person or thing.
It is not good that man should be alone. Gen 2.
[This adjective follows its noun.]
2. It is applied to two or more persons or things, when separate from others, in a place or condition by themselves; without company.
And when they were alone, he expounded all things to his disciples. Mark 4.
Thou whose name alone is Jehovah. Psa 83.
This sense at first appears to be adverbial, but really is not; whose name single, solitary, without another, is Jehovah.
To let alone is to suffer to rest; to forbear molesting or meddling with; to suffer to remain in its present state. Alone, in this phrase, is an adjective, the word to which it refers being omitted; let me alone; let them alone; let it alone; that is, suffer it to be unmolested, or to remain as it is, or let it remain by itself.
ALO'NE, adv. Separately; by itself.