Perseverance is undoubtedly an important aspect of successful entrepreneurship. The saying "If at first you don't succeed, try, try again" means that few individuals are able to achieve great things without first overcoming the obstacles that stand in their way.

Here are four examples - two from the past and two from the present day - of successful perseverance in business to help inspire you to achieve the seemingly impossible.

Thomas Edison

When he was young, Thomas Edison's parents took him out of school after his teachers declared that he was "stupid" and "unteachable." Edison spent his early years working and being fired from various jobs, culminating in his firing from a telegraph company at the age of 21. Despite these numerous setbacks, he Edison was never discouraged from his true calling in life: inventing. Throughout his career, Edison obtained more than one thousand patents. And although several of these inventions -- such as the light bulb, stock printer, phonograph and alkaline battery -- were groundbreaking innovations, the vast majority of them could be fairly described as failures. And now Edison is famous for saying that genius is "1% inspiration and 99% perspiration."

One of Edison's best examples of perseverance occurred after he was already a successful man. After inventing the light bulb, he began seeking inexpensive light bulb filament. At the time, ore was mined in the Midwest of the United States, and shipping costs were very high. In order to combat this, Edison established his own ore-mining plant in Ogdensburg, New Jersey. For nearly ten years, he devoted his time and money to the enterprise. Edison also obtained 47 patents for innovations that helped make the plant run more smoothly. And even despite those inventions, Edison's core project failed because of low quality ore on the East Coast.

However, despite that failing, one of those 47 inventions (a crushing machine) revolutionized the cement industry, and actually earned Edison back almost all of the money he lost. Later, Henry Ford would credit Edison's Ogdensburg project as the main inspiration for his Model T Ford assembly line. And in fact, many believe that Edison paved the way for modern-day industrial laboratories. Edison's foray into ore-mining demonstrates that dedication can pay off even in a losing venture.

Milton Hershey

Milton Hershey had a long path to the top of the chocolate industry. Hershey dropped out of the 4th grade to take an apprenticeship with a printer, only to be fired. Next he became an apprentice to a candy-maker, and then started 3 unsuccessful candy enterprises.

However, Hershey was not giving up. After these unsuccessful attempts, he founded the Lancaster Caramel Company. Despite his initial setbacks, Hershey's caramel recipe was a huge success. Looking beyond caramel, Hershey believed that chocolate products had a much greater future, and sold the Lancaster Caramel Company in order to start the Hershey Company, which brought milk chocolate to the masses.

In doing so, Hershey overcame failure and accomplished his goals. He also created hundreds of jobs for Pennsylvanians and was generous with his wealth, building houses, churches, and schools.

Steve Jobs

Perseverance is not just limited to the beginning phases of a person's career. In fact, failure can often occur after a long period of achievement.

Apple founder Steve Jobs achieved phenomenal success early in life. When he was 20 years old, he founded Apple from his parents' garage, and within ten years the company had grown into a $2 billion juggernaut. However, when Jobs turned 30, Apple's Board of Directors fired Jobs from the company he created, and he found himself unemployed. Rather than seeing this as a curse, Jobs treated it as a freedom to pursue new initiatives. In fact, Jobs later stated that being fired was one of the best things that ever happened to him, since it provided him with the opportunity to think more creatively and to start a new company.

After being fired from Apple, Jobs founded NeXT, a software company, and Pixar, the amazing movie company that has produced animated films such as Finding Nemo. NeXT was subsequently purchased by Apple. After founding these companies, Jobs not only went back to Apple, but he helped launch their current resurgence in popularity with the creation and success of the iPod and iPhone. Jobs credits his career success and his strong relationship with his family to the fact that he was terminated from Apple.

Simon Cowell

Although Simon Cowell is now a pop icon and wealthy man, Cowell faced struggles earlier in life. When he was fifteen, he dropped out of school and worked various odd jobs. Cowell eventually received a job working in the mail room at EMI Music Publishing, where he was able to work his way into the A&R department. After EMI, Cowell formed his own publishing company, E&S Music.

Unfortunately, Cowell's new company folded in its first year of operation. As a result, Cowell was burdened with a lot of debt, and had to move back in with his parents. However, he was persistent, and eventually landed a job with a small company called Fanfare Records. Cowell worked at Fanfare for eight years and was able to help build the company into a successful record label. From there, he spent several years signing musicians and cultivating talent before launching the "American Idol" and "X-Factor" franchises that would make Simon Cowell a household name.