Locke attacks both the view that we have any innate principles (for example, the whole is greater than the part, do unto others as you would have done unto you, etc.) as well as the view that there are any innate singular ideas (for example, God, identity, substance, and so forth). The main thrust of Locke’s argument lies in pointing out that none of the mental content alleged to be innate is universally shared by all humans. He notes that children and the mentally disabled, for example, do not have in their minds an allegedly innate complex thought like “equals taken from equals leave equals”. He also uses evidence from travel literature to point out that many non-Europeans deny what were taken to be innate moral maxims and that some groups even lack the idea of a God. Locke takes the fact that not all humans have these ideas as evidence that they were not implanted by God in humans minds, and that they are therefore acquired rather than innate.
Consider what it is like if you are staffing an intellectual property law firm. From time to time you have varying percentages of different kinds of work. For a year or two, you have mostly patent, let's say, and only a little copyright. Then for a couple of years a big copyright lawsuit or two consumes a large portion of the firm's personnel. Then it shifts back again. If all the lawyers in the firm have techie backgrounds and are admitted to practice before the Patent Office, then you can effortlessly accommodate these shifts. The patent lawyers can easily enough do the copyright work, etc. If, on the other hand, the firm hired say half techies and half non-techies, then you have times when there is not enough work to keep all the non-techies fully billable. The firm's profitability suffers, or the non-techies get laid off. After a couple cycles of this, the firm's partners catch on that there is a drawback to hiring non-techies.