Jana Partners' Barry Rosenstein is still bullish on the stock market.

The activist hedge fund manager was asked what he thought about recent comments from Appaloosa Management's David Tepper saying the market is "cheap."

"I agree with David completely. In fact we are more invested today than we've ever been," Rosenstein said Tuesday on CNBC's "Fast Money Halftime Report." "We have found more ideas that meet our threshold than any other time."

The investor shared his key reasons on why he is optimistic on equities.

"The economy is growing. Earnings are growing. Rates are at all-time lows. It just seems like the market [rally] is going to continue for awhile," he added.

Rosenstein is founder and managing partner of Jana Partners. The firm has $5 billion of assets under management.

Jana Partners and the California State Teachers' Retirement System, or CalSTRS, also urged Apple take further action to allow parents more options to limit children's phone use.