Non-static method error when trying to use class in widgets

I’m trying to call a method into a function widget. The $this of the widget is passed to that method, but I get the following error:

Strict Standards: Non-static method class_name::method_name() should not be called statically, assuming $this from incompatible context in C:xampphtdocswpwp-contentthemes...... on line 446

Here’s what I’m trying to do exactly.

The widget form function:

class new_widget extends WP_Widget {
    function form($instance) {
        <?php widget_fields::input($instance, 'title'); ?>
    }
}

And the class which I’m using out of the score of the widget class:

class widget_fields {
    function input($t, $slug) {
        ?>
        <p>
            <input type="text" name="<?php echo $this->get_field_name($slug); ?>" value="<?php echo $t[$slug]; ?>" />          
        </p>
        <?php

    }
}