"you can invest your savings. If you ain´t got any, you ain´t gonna invest". (I admit you could argue that you can borrow but still nobody ain´t lending ya if you ain´t gonna givvet baksh).
Borrowing for investing is usually a bad decision. One must invest only money he affords to lose. The only "good" way to borrow for investing is if you have plenty of wealth, but it's already locked (properties, companies) and borrowing will provide liquidity.
Also for normal people, I'd suggest to invest
only part of the savings, not all. Some should stay for unexpected problems.
However, these are (small) details. The initial rule stands.