The plot thickens

From John Locke, Some Thoughts Concerning Education (1693):

We must not hope wholly to change their Original Tempers, nor make the Gay Pensive and Grave, nor the Melancholy Sportive, without spoiling them. God has stampt certain Characters upon Mens Minds, which, like their Shapes, may perhaps be a little mended; but can hardly be totally alter’d, and transform’d into the contrary.

...

These native Propensities, these prevalencies of Constitution, are not to be cured by Rules, or a direct Contest; especially those of them that are the humbler and meaner sort, which proceed from fear, and lowness of Spirit; though with Art they may be much mended, and turned to good purposes. But this, be sure, after all is done, the Byass will always hang on that side, that Nature first placed it: And if you carefully observe the Characters of his Mind, now in the first Scenes of his Life, you will ever after be able to judge which way his Thoughts lean, and what he aims at, even hereafter, when, as he grows up, the Plot thickens, and he puts on several Shapes to act it.