$\def \u#1{\,\mathrm{#1}}$ $\def \abs#1{\left|#1\right|}$ $\def \ast{*}$ $\def \deg{^{\circ}}$ $\def \ten#1{\times 10^{#1}}$ $\def \redcancel#1{{\color{red}\cancel{#1}}}$ $\def \BLUE#1{{\color{blue} #1}}$ $\def \RED#1{{\color{red} #1}}$ $\def \PURPLE#1{{\color{purple} #1}}$ $\def \th#1,#2{#1,\!#2}$ $\def \lshift#1#2{\underset{\Leftarrow\atop{#2}}#1}}$ $\def \rshift#1#2{\underset{\Rightarrow\atop{#2}}#1}}$ $\def \dotspot{{\color{lightgray}{\circ}}}$

Conservation of Charge

The reason Franklin described charge as "positive" and "negative", instead of something like "red" and "blue", is because the two types of charge cancel out: if you place $q=-4\u{nC}$ on an object and then $q=+4\u{nC}$ on the same object, the total charge on the object is $q=0$. (We say that the object is neutral in that case.) This is a consequence of an important law:

Law of Charge Conservation
If the charge of a system changes,
it is due to the flow of charge into or out of the system.
Charge cannot be created or destroyed.

Thus an object can become less positive only if positive charge flows out of it OR if negative charge flows into it: it can't just "become" more or less positive without that flow.