If every entrepreneur shredded the business plan after being told there wasn’t room in the market for his or her idea, the industry would have missed some great innovations. I wonder how many times the founders of Google were told the Internet doesn’t need another search engine or that Juniper Networks was misguided in taking on Cisco because it already “owned” the router space. In my own case, when founding both NetScreen and Fortinet, I was told that the network security market had “matured” and didn’t need another groundbreaking idea.

Six years ago, firewalls were supposedly the panacea in security, and the market was both dominated and nearing saturation. The same, I was told, was true for other security provisions, including VPNs, antivirus, and intrusion detection systems (IDS). But at the time, no one was solving the perplexing and growing problem of defending against blended threats, and the plethora of so-called “best-of-breed” solutions were functionally deficient, resource-intensive and cost-prohibitive.

A startup has the twin advantages of being able to spot an opportunity early on, and to execute much faster than larger companies can. Both advantages are rooted in the “legacy thinking” that often plagues companies with existing solutions and customers. The best entrepreneurs, therefore, are those who can see the world with a fresh perspective, unencumbered by “group think,” and possess the confidence of their convictions to persist against the odds.

When founding Fortinet, for example, our vision was an integrated network and content security solution that could keep pace with Internet evolution and ever-changing threats without crippling network performance or administrative resources. That idea—involving the integration of a custom content processor with many home-grown, interrelated security technologies and services—required a significant investment in R&D for such a young company. However, we knew that innovation would be the driver for success. Taking that risk prompted an entirely new market: Unified Threat Management, or UTM.

Of course, the naysayers said it couldn’t be done. “The field is too specialized.” “A multi-function security appliance won’t be able to perform at LAN speeds.” “The barriers to entry are just too high.” And my favorite: “There isn’t room for another security vendor.”

Clearly, they were wrong. Fortinet now leads the UTM market—the fastest-growing segment in the network security industry.

Like others before and after us, Fortinet is successful today because we were able to stand resilient against the army of critics, stay committed to our vision, and relentlessly execute on our business plan. While successful entrepreneurs must take calculated risks, they must also never forget the basics—believe, focus and execute.

When a start-up matures and succeeds, it faces an array of new and different challenges—among them, how to carry on the entrepreneurial spirit, nimbleness and vision as its employee, customer and revenue base grows. I’ve personally found it important to shun the aforementioned legacy and group-think mentalities and never lose sight of the innovation that got us a seat at that said crowded table.

While some would contend that the market once again has too many players and diminishing differentiation, a good entrepreneur looks around and sees amazing opportunity and innovation brewing.

Squeezing a fresh idea into an established market is no easy task, but when an entrepreneur mixes resilience and perseverance with business discipline and innovation, the road to success is easier to travel.