As discussed earlier, Goldman's entire S&P500 price forecast for 2018 and the next three years is based on two things: tax reform passing, but more broadly, something that David Kostin dubbed "Rational Exuberance", to wit:

"Rational exuberance" best describes our forecast for the trajectory of the S&P 500 during the next several years. Earnings drive stocks over time and should support the index rising to 2850 at year-end 2018, 3000 at the end of 2019, and 3100 by the close of 2020, representing a price gain during the next three years of 20%. Our price targets imply a modest expansion in forward P/E multiple to 18.2x at year-end 2018, a flat multiple in 2019, and a contraction to 18.1x in 2020.

As Kostin describes it, "rational exuberance" is defined by "above-trend US and global economic growth, low inflation, low albeit slowly rising interest rates, and underlying corporate profits boosted by pending corporate tax reform likely to be adopted by early next year."

So far so good, but as Kostin also explained, absent tax reform passing, the S&P will not only not hit 3,100 in 3 years, it may well be lower: "Assuming tax reform passes, we forecast S&P 500 adjusted EPS will jump by 14% to $150 in 2018. Equity investors will be rewarded as the index advances by 11% to 2850 at year-end 2018 and delivers a total return of 13% including the 2% dividend yield."