I cringe at the cookie-cutter replies to this question. There is no right answer. It depends on the stage of your business and who the investor is.

If you're at seed stage and an individual angel investor is asking for 40%, sure, that is a deal breaker and you won't be "investible" afterwards as your cap table structure will be screwed. So that's a no, in principle. Even then, it's not a hard no. Do you really love your company and want to lead it during the next 10 years, or are you comfortable with answering to someone else and maybe even leaving after a few years? These are radically different situations. And even if you love your company, if you don't have a choice you might have to take the deal. But search for better terms from someone else first.

If, on the other hand, your company is bigger (Series B, Series C, etc) then it might be useful to bring in someone for a large chunk of equity - a venture capital, for example. Just be aware of the perils of this, which most founders aren't. Venture capitals are affected by whims (personal investor problems, need to give returns on a given year, and so many more random factors) and they will probably polarize your business model - they will try and force you to go for a billion-dollar exit with less chances than a reasonably profitable company - because their whole business model is based on that.

Give more info on the specific investors, your vertical, situation, financials and other terms you've obtained and I can give a more in-depth answer.