Maximum function for all numbers in a list

# MAXIMUM FUNCTION FOR ALL NUMBERS IN A LIST WITH N ENTRIES

num_list = [1, 4, 5, 100, "Text", 3.5, 23]

def maxN(list):
    # Create a new list which only includes integers and floats from old list:
    list_num_only = [k for k in list if isinstance(k, float) or isinstance(k, int)]
    # Define the maximum of 2 numbers:
    def max(a, b):
        return (list_num_only[a] + list_num_only[b] + abs(list_num_only[a] - list_num_only[b])) / 2
    max_interim = 0
    for i in range(len(list_num_only)-1):
        max_interim = max(max(list_num_only[i], list_num_only[i+1]), list_num_only[i+2])
    return max_interim

print(maxN(num_list))